I've never used Rhapsody but all rockbox cares about is having files on the disk, so as long as they end up there it should work, though you may need to use the database in rockbox to actually find where Rhapsody stores the files . . .
Installing Rockbox does not change how Rhapsody or the OF work. They're still work with the same files they did before, which is probably just MP3/WMA/AAC/WAV.