APFS container over multiple physical disks
There is no official documented way to add another physical storage to an existing APFS container.

I tried several possibly "undocumented" apfsVerbs like addPhysical, addPhysicalStorage or addStorage without success.

The feature may still be implemented later, since creating APFS FusionLCs is possible already:

diskutil ap createContainer [-main] device [-secondary] [device]


Example:

$ diskutil ap createContainer disk1s2 disk2s2    
Creating container with disk1s2 disk2s2
Started APFS operation on disk1s2 Data
Creating a new empty APFS Container
Unmounting Volumes
Switching disk1s2 to APFS
Switching disk2s2 to APFS
Creating APFS Container
FusionLC autodetect: LC Fusion
Created new APFS Container disk4
Disk from APFS operation: disk4
Finished APFS operation on disk1s2 Data

$ diskutil ap addVolume disk4 APFS Data
Exporting new unencrypted APFS Volume "Data" from APFS Container Reference disk4
Started APFS operation on disk4
Preparing to add APFS Volume to APFS Container disk4
Creating APFS Volume
Created new APFS Volume disk4s1
Mounting APFS Volume
Setting volume permissions
Disk from APFS operation: disk4s1
Finished APFS operation on disk4