Even easier…use your other software to export an .m3u playlist referring to the files for each number of stars.
Roon will import the playlist for you. Now you have an easy way to get a handle on the tracks and manipulate them.
Make a tag for each number of stars, and move tracks over into the tag once.
If you name the tags using ★ characters and it will look something like this:
You could use the same approach for moving over other stuff like tags or playlists constructed in other software.
Yes, not totally automatic, but not as impossible as some of the discussion in this thread suggests either.