So long as the music is on your phone, you can add it while editing the movie on the phone:
Select the project you want to add the music to.
Click on the 'import media' button (the bottom left button , looks like a box with a down arrow in it)
There are three buttons at the bottom of the screen, Video, Photo's and Audio, select Audio.
You should now have access to the music and playlists in your phones iPod.
Browse to the song you want to overlay and tap on it.
The song replaces the existing audio track if you have theme audio on (you toggle theme audio by clicking on the gear wheel at the very start of your movies timeline) .
If theme audio is off the track you selected is inserted under the timeline.
Music, whether from the theme or from your iPod is shown as a green bar under the timeline.
AKAIK you can only insert one track and you can't make it loop. If your movie is long you may have to extend the track outside of iMovie before using it.
HTH