Let's say that you've used MusicBee to organize your songs and now you want to export your ratings into iTunes. MusicBee saves your ratings to the original files, but iTunes doesn't read this information. Here's a workaround:
- Open MusicBee and create 5 auto-playlists: R1, R2, R3, R4, R5 that restrict the library to the songs that have the rating value 1, 2, 3, 4 and 5.
- Export these playlists to iTunes: right-click playlist, "List> Export as External Playlist", then import the M3U into iTunes. See details here: 
- Open iTunes and create a new standard playlist: Temp.
- Select all the songs from the playlist R1 and drag them to the Temp playlist in iTunes. Select all the songs from Temp, right-click, select "Rating", choose 1 star and then delete all the songs from the playlist.
- Repeat step 3 for the other playlists: R2, R3, R4, R5.