your best bet is to change your settings to add the music to library when played. then you need to change the import settings to import as mp3, quality as desired.
now you need to find the music in windows (mac os), and open it. it will then play in iTunes and add it to your library as an mp3. delete the original.
(make sure it works first)
i've never had to do it, but theoretically, it should work just fine