Combine Time Machine backups in Mojave
I think I've found a workaround (or "ridiculous hack", if you will). You can't modify Backups.backupdb, but you can delete it and replace it (back up your backups, if you feel the need):


Create a folder named "WhosTheBoss"
Copy any backups you want to keep in there (same directory structure)
Delete the original Backups.backupsdb folder
Rename "WhosTheBoss" to "Backups.backupsdb"
Point Time Machine and/or Migration Assistant to that location and win.