Cool. All I really wanted to do initially was beef up Carthage, and it looks like we can do that as long as we don't try to give them any provinces that belong to a Roman faction. In the process, the most complicated thing you have to do is move or delete the previous owner's units, and as long as you don't delete their Family Members, it works.

If anyone ever figures out how to move Roman factions in the same way, or even just why it doesn't work, please let me know.