Asign unallocated free space on external volume to (new) partition OSX
The procedure posted in this answer relies on the use of the command gdisk. The command is not included with macOS. Included is the procedure for installing this command.


  Note: Disk identifiers (disk2 for example) and partition identifiers (disk2s1 for example) can be set arbitrarily by macOS. I tried to post the correct identifiers. Before executing a command, you may want to review the output from diskutil list to insure the correct identifier is being used. 


Install gdisk


Download gdisk from the internet. I assume this will be downloaded to your Downloads folder.
Execute the command given below to remove the quarantine.

xattr -d com.apple.quarantine ~/Downloads/gdisk-1.0.4.pkg

Use the Finder application to install gdisk.


Convert a MBR Partition Table to a GUID Partition Table (GPT).


  Note: Not all drives using a MBR Partition Table can be converted to use a GUID Partition Table (GPT). The drive posted in the OP's question happens a drive that can be  easily converted.



Use command given below to determine the identifier for the drive. I will assume this is disk2, as shown in your question.

diskutil list

Execute the following commands to perform the conversion.

diskutil unmountdisk disk2
gdisk /dev/disk2
w
y


Below is an example.

$ diskutil unmountdisk disk2
Unmount of all volumes on disk2 was successful
$ gdisk /dev/disk2
GPT fdisk (gdisk) version 1.0.4

Warning: Devices opened with shared lock will not have their
partition table automatically reloaded!
Partition table scan:
  MBR: MBR only
  BSD: not present
  APM: not present
  GPT: not present


***************************************************************
Found invalid GPT and valid MBR; converting MBR to GPT format
in memory. THIS OPERATION IS POTENTIALLY DESTRUCTIVE! Exit by
typing 'q' if you don't want to convert your MBR partitions
to GPT format!
***************************************************************


Command (? for help): w

Final checks complete. About to write GPT data. THIS WILL OVERWRITE EXISTING
PARTITIONS!!

Do you want to proceed? (Y/N): y
OK; writing new GUID partition table (GPT) to /dev/disk2.
Warning: Devices opened with shared lock will not have their
partition table automatically reloaded!
Warning: The kernel may continue to use old or deleted partitions.
You should reboot or remove the drive.
The operation has completed successfully.

Eject the drive by executing the command given below.

diskutil eject disk2

Unplug the drive and plug back in. Execute the command below to see if the drive identifier has changed.

diskutil list



Add an EFI Partition


  Note: The defaults shown in the examples may not match what you will see.



Create the partition. Selecting a partition type of Linux prevents the partition from mounting before being formatted.

diskutil unmountdisk disk2
gdisk /dev/disk2
s
n
3
40
409639
8300
s
w
y


Below is an example.

$ diskutil unmountdisk disk2
Unmount of all volumes on disk2 was successful
$ gdisk /dev/disk2
GPT fdisk (gdisk) version 1.0.4

Warning: Devices opened with shared lock will not have their
partition table automatically reloaded!
Partition table scan:
  MBR: protective
  BSD: not present
  APM: not present
  GPT: present

Found valid GPT with protective MBR; using GPT.

Command (? for help): s
You may need to edit /etc/fstab and/or your boot loader configuration!

Command (? for help): n
Partition number (2-128, default 2): 3
First sector (34-3906250006, default = 40) or {+-}size{KMGTP}: 40
Last sector (40-409639, default = 409639) or {+-}size{KMGTP}: 409639
Current type is 'Apple HFS/HFS+'
Hex code or GUID (L to show codes, Enter = AF00): 8300
Changed type of partition to 'Linux filesystem'

Command (? for help): s
You may need to edit /etc/fstab and/or your boot loader configuration!

Command (? for help): w

Final checks complete. About to write GPT data. THIS WILL OVERWRITE EXISTING
PARTITIONS!!

Do you want to proceed? (Y/N): y
OK; writing new GUID partition table (GPT) to /dev/disk2.
Warning: Devices opened with shared lock will not have their
partition table automatically reloaded!
Warning: The kernel may continue to use old or deleted partitions.
You should reboot or remove the drive.
The operation has completed successfully.

Eject the drive by executing the command given below.

diskutil eject disk2

Unplug the drive and plug back in. Execute the command below to see if the drive identifier has changed.

diskutil list

FAT32 format the partition.

newfs_msdos -F 32 -v EFI /dev/rdisk2s1


Below is an example.

$ newfs_msdos -F 32 -v EFI /dev/rdisk2s1
512 bytes per physical sector
/dev/rdisk2s1: 403266 sectors in 403266 FAT32 clusters (512 bytes/cluster)
bps=512 spc=1 res=32 nft=2 mid=0xf8 spt=32 hds=32 hid=40 drv=0x80 bsec=409600 bspf=3151 rdcl=2 infs=1 bkbs=6

Change the partition type to EFI.

diskutil unmountdisk disk2
gdisk /dev/disk2
t
1
EF00
w
y


Below is an example.

$ diskutil unmountdisk disk2
Unmount of all volumes on disk2 was successful
$ gdisk /dev/disk2
GPT fdisk (gdisk) version 1.0.4

Warning: Devices opened with shared lock will not have their
partition table automatically reloaded!
Partition table scan:
  MBR: protective
  BSD: not present
  APM: not present
  GPT: present

Found valid GPT with protective MBR; using GPT.

Command (? for help): t
Partition number (1-2): 1
Current type is 'Linux filesystem'
Hex code or GUID (L to show codes, Enter = AF00): EF00
Changed type of partition to 'EFI System'

Command (? for help): w

Final checks complete. About to write GPT data. THIS WILL OVERWRITE EXISTING
PARTITIONS!!

Do you want to proceed? (Y/N): y
OK; writing new GUID partition table (GPT) to /dev/disk2.
Warning: Devices opened with shared lock will not have their
partition table automatically reloaded!
Warning: The kernel may continue to use old or deleted partitions.
You should reboot or remove the drive.
The operation has completed successfully.



Add a New Mac OS Extended (Journaled) Volume

The free space before the original two volumes can not be added to either of these volumes. To reclaim this space, one or more new volumes need to be created. The procedure below creates one new volume.


Create the partition. Selecting a partition type of Linux prevents the partition from mounting before being formatted.

diskutil unmountdisk disk2
gdisk /dev/disk2
n
4
409640
69999999
8300
s
w
y


Below is an example.

$ diskutil unmountdisk disk2
Unmount of all volumes on disk2 was successful
$ gdisk /dev/disk2
GPT fdisk (gdisk) version 1.0.4

Warning: Devices opened with shared lock will not have their
partition table automatically reloaded!
Partition table scan:
  MBR: protective
  BSD: not present
  APM: not present
  GPT: present

Found valid GPT with protective MBR; using GPT.

Command (? for help): n
Partition number (2-128, default 2): 4
First sector (34-3906250006, default = 409640) or {+-}size{KMGTP}: 409640
Last sector (409640-3906250006, default = 3906250006) or {+-}size{KMGTP}: 69999999
Current type is 'Apple HFS/HFS+'
Hex code or GUID (L to show codes, Enter = AF00): 8300
Changed type of partition to 'Linux filesystem'

Command (? for help): s
You may need to edit /etc/fstab and/or your boot loader configuration!

Command (? for help): w

Final checks complete. About to write GPT data. THIS WILL OVERWRITE EXISTING
PARTITIONS!!

Do you want to proceed? (Y/N): y
OK; writing new GUID partition table (GPT) to /dev/disk2.
Warning: Devices opened with shared lock will not have their
partition table automatically reloaded!
Warning: The kernel may continue to use old or deleted partitions.
You should reboot or remove the drive.
The operation has completed successfully.

Eject the drive by executing the command given below.

diskutil eject disk2

Unplug the drive and plug back in. Execute the command below to see if the drive identifier has changed.

diskutil list

Format the partition Mac OS Extended (Journaled). This will also change the partition type to the correct value.

diskutil erasevolume jhfs+ Name3 disk2s2


Below is an example.

$ diskutil erasevolume jhfs+ Name3 disk2s2
Started erase on disk2s2 My2TB
Unmounting disk
Erasing
Initialized /dev/rdisk2s2 as a 33 GB case-insensitive HFS Plus volume with a 8192k journal
Mounting disk
Finished erase on disk2s2 Name3



Reclaim the Remaining Free Space

Add the remaining free space to the existing Volumes. Any free space that is remaining after entering these commands should be fairly small and is required to be reserved for use by macOS. 

    diskutil resizevolume disk2s2 R
    diskutil resizevolume disk2s3 R
    diskutil resizevolume disk2s4 R


Below is an example.

    $ diskutil resizevolume disk2s2 R
    Unable to find disk for disk2s2
    $ diskutil resizevolume disk2s2 R
    Resizing to full size (fit to fill)
    Started partitioning on disk2s2 Name3
    Verifying the disk
    Verifying file system
    Volume was successfully unmounted
    Performing fsck_hfs -fn -x /dev/rdisk2s2
    Checking Journaled HFS Plus volume
    Checking extents overflow file
    Checking catalog file
    Checking multi-linked files
    Checking catalog hierarchy
    Checking extended attributes file
    Checking volume bitmap
    Checking volume information
    The volume Name3 appears to be OK
    File system check exit code is 0
    Restoring the original state found as mounted
    Resizing
    Modifying partition map
    Growing file system
    Finished partitioning on disk2s2 Name3
    /dev/disk2 (external, physical):
       #:                       TYPE NAME                    SIZE       IDENTIFIER
       0:      GUID_partition_scheme                        *2.0 TB     disk2
       1:                        EFI EFI                     209.7 MB   disk2s1
       2:                  Apple_HFS Name3                   199.8 GB   disk2s2
       3:                  Apple_HFS Name1                   200.2 GB   disk2s3
       4:                  Apple_HFS Name2                   200.2 GB   disk2s4
    $ diskutil resizevolume disk2s3 R
    Resizing to full size (fit to fill)
    Started partitioning on disk2s3 Name1
    Verifying the disk
    Verifying file system
    Volume was successfully unmounted
    Performing fsck_hfs -fn -x /dev/rdisk2s3
    Checking Journaled HFS Plus volume
    Checking extents overflow file
    Checking catalog file
    Checking multi-linked files
    Checking catalog hierarchy
    Checking extended attributes file
    Checking volume bitmap
    Checking volume information
    The volume Name1 appears to be OK
    File system check exit code is 0
    Restoring the original state found as mounted
    Resizing
    Modifying partition map
    Growing file system
    Finished partitioning on disk2s3 Name1
    /dev/disk2 (external, physical):
       #:                       TYPE NAME                    SIZE       IDENTIFIER
       0:      GUID_partition_scheme                        *2.0 TB     disk2
       1:                        EFI EFI                     209.7 MB   disk2s1
       2:                  Apple_HFS Name3                   199.8 GB   disk2s2
       3:                  Apple_HFS Name1                   1.2 TB     disk2s3
       4:                  Apple_HFS Name2                   200.2 GB   disk2s4
    $ diskutil resizevolume disk2s4 R
    Resizing to full size (fit to fill)
    Started partitioning on disk2s4 Name2
    Verifying the disk
    Verifying file system
    Volume was successfully unmounted
    Performing fsck_hfs -fn -x /dev/rdisk2s4
    Checking Journaled HFS Plus volume
    Checking extents overflow file
    Checking catalog file
    Checking multi-linked files
    Checking catalog hierarchy
    Checking extended attributes file
    Checking volume bitmap
    Checking volume information
    The volume Name2 appears to be OK
    File system check exit code is 0
    Restoring the original state found as mounted
    Resizing
    Modifying partition map
    Growing file system
    Finished partitioning on disk2s4 Name2
    /dev/disk2 (external, physical):
       #:                       TYPE NAME                    SIZE       IDENTIFIER
       0:      GUID_partition_scheme                        *2.0 TB     disk2
       1:                        EFI EFI                     209.7 MB   disk2s1
       2:                  Apple_HFS Name3                   199.8 GB   disk2s2
       3:                  Apple_HFS Name1                   1.2 TB     disk2s3
       4:                  Apple_HFS Name2                   598.6 GB   disk2s4


Appendix

Descriptions for the gdisk commands is given below.

b   back up GPT data to a file
c   change a partition's name
d   delete a partition
i   show detailed information on a partition
l   list known partition types
n   add a new partition
o   create a new empty GUID partition table (GPT)
p   print the partition table
q   quit without saving changes
r   recovery and transformation options (experts only)
s   sort partitions
t   change a partition's type code
v   verify disk
w   write table to disk and exit
x   extra functionality (experts only)
?   print this menu