I don't know how its done in Garageband but,I imported a couple of midi song files into GTP 4.I made a folder in GTP for midi songs and saved the song files I down loaded there.Then,I opened a project in GTP 4,found the folder and just dragged the files into the track pane of my project,GTP 4 created all of my midi and synth tracks for me.I'm sure there is a better way to do it.