using a previous recorded song for jam session

I wanted to record some new tracks in a previously recorded song using the jam session mode. The song was on my iPad, but in the jam session the other ipad user was to be the 'master' to which all existing tracks and new tracks should be transferred after recording.

This appeared to be impossible, as I was supposed to start a new song.

Is this not possible at all, or was this the case because the existing tracks were real audio tracks?