It doesn't look, as if there is an option to import a song into Garageband directly. Garageband doesn't have an option to import songs, and if you add an mp3 file via iTunes to Garageband, it doesn't show up in Garageband. So it seems the only option is to re-record the song with Garageband, meaning you would play the song on another device and then use Garageband to record it.
If there is a direct way, it hasn't crossed my path, yet.