Using the Jukebox, you can convert existing .WAV files into compressed .MP3 files. The old .WAV file will not be deleted unless you choose to delete it separately.
These compressed .MP3 files will be added into the database as with any other .MP3 song.
Go to Add Songs From Disk. Select the .wav (or any other) file(s) you want to add to your database, and click ok.