Okay for starters, I have just recently bought and installed iLife, including the new GarageBand, onto my computer. The first day I played around with it and was very impressed with the ease of use of the app. I had already compiled some songs in a different app about a month ago, and wanted to use the AIFF files I had on my hard driive with GarageBand. To my surprise, there was no "Import audio from file" option in the file menu.
Looking at my GarageBand document "My Song," I noticed it was a package whose contents could be observed. Inside I found a folder with audio files from some vocal recordings I was testing. The names were Recording#01.aif, Recording#02.aif, etc. The extension on each of them were .aif, so this gave me an idea. I decided to replace one of these aif files with my precompiled song. Sure enough, when I re-opened "My Song" in GarageBand, the song was played instead of the vocal recording. So for the recap:
Mac OS X Hints
http://hints.macworld.com/article.php?story=20040118174548168