[ashlar]

Yes you can do that.

Go to any movie in library and open Manage versions on context, select Add versions then navigate to file system location of any version you have, no need to use convert at all.

Yes, I know you can. This is why I added that it's counterintuitive to consider the way you chose, while handling a library based system.
If I already have movies in the library, no way it makes sense to navigate in folders to search for a different version. I mean, Kodi has spent years educating users to the library approach. I'm not saying you did wrong in doing what you did. Just that it's... "wrong". :-)