The easy way would be to drop them into the Shared folder, or the 'Documents' folder in iTunes File Sharing. Take a look at this post:
http://forum.harmonicdog.com/viewtopic.php?f=2&t=195
Another way which should reduce file space usage would be to upload them directly into a song, using Wi-Fi:
http://forum.harmonicdog.com/viewtopic.php?f=2&t=4
In either case, you'll need to copy/paste them onto tracks. You can access them in MultiTrack by Long-Touch over a track and select "Bins", then the "Shared" tab, then the "Copy" button. Now, Long-Touch over a track again and select "Paste".
