Time Machine - How does one convert Backups.backupdb to a backupbundle?
While I haven't tried this personally, I recently came across a workaround posted on the MacRumors forums that claims it should work:

There is a work around to do this. Attach the drive to the router
(assuming it supports Time Machine) then start a backup to the disk
using Time Machine. Give it a couple minutes to get started, then stop
it and disconnect.
Now attach the drive to the Mac over USB. Look in Finder and you will
see a sparse bundle with the partial Backups.backupdb file there. Just
delete that file and empty the trash.
Now turn on Time Machine and do a full backup. When it is done you
will have the full Backups.backupdb on the drive. Just move that from
the root of the drive into the sparse bundle. Now connect it back to
the router and turn TM back on. It will pick up where it left off with
incremental backups going forward.

Another variation on this method creates a local Samba file share mount on the USB drive as a workaround. Full details in the linked answer, but a summary snippet follows:

sudo ifconfig lo0 alias 127.0.0.2/32
Plug your usb drive, then via System Preferences / Sharing add a smb share to a folder time-machine-macbook in the drive time-machine-usb Then, add a destination backup (Time Machine will see it as a network share)
sudo tmutil setdestination -a &quot;smb://user:password@127.0.0.2/time-machine-macbook&quot;

If there is a simpler/more direct way to do it though, that would be awesome to know! I was thinking maybe something in the tmutil command line program may be able to do it, but haven't yet tried.
According to Apple's Disk Utility User Guide, it appears possible to manually create a sparsebundle disk image:


In the Disk Utility app  on your Mac, choose File &gt; New Image &gt; Blank Image
Enter a filename for the disk image, add tags if necessary, then
choose where to save it.
In the Name field, enter the name for the disk image.
In the Size field, enter a size for the disk image.
Click the Format pop-up menu, then choose the format for the disk:

I believe for Time Machine you will need to choose Mac OS Extended (Journaled) or Mac OS Extended (Case-sensitive, Journaled) (reference)


To encrypt the disk image, click the Encryption pop-up menu, then
choose an encryption option.
Click the Partitions pop-up menu, then choose a partition layout.
Click the Image Format pop-up menu, then choose an option:

Sparse bundle disk image: Same as a sparse disk image (below), but the
directory data for the image is stored differently. Uses the
.sparsebundle file extension.
Sparse disk image: Creates an expandable file that shrinks and grows
as needed. No additional space is used. Uses the .sparseimage file
extension.


Click Save, then click Done.
Disk Utility creates the disk image file where you saved it in the
Finder and mounts its disk icon on your desktop and in the Finder
sidebar.
In the Finder, copy your files to the mounted disk image, then eject
it.


Edit Note: macOS Catalina appears to use the extension backupbundle instead of sparsebundle, but it seems that this file is still mountable in the same ways as previously.