You can't add more than one song to a themed video.
However, you can either set up your video to have no music or delete the music after it's created -- and then just add your own AFTER the InstantMovie is created by applying it to the Soundtrack audio track below it on the timeline.
Thanks Steve. The documentation for PRE 9 seems to imply we should be
able to do this though. It states "Music - Specifies whether to use the
theme music, your own music (click Browse to locate and open it), or no
music. You can select multiple music clips (you can select them here or
from the Organize workspace as part of the original asset selection),
and they are played in the order selected."
As you suggested, I thought about deleting it afterwards, then adding my
own music. Though the documentation also states that the results of the
project will change, based on the beat or the temple of the chosen
music. I'm curious how much of a difference that would make...
Oh well. Thanks again Steve.