Unfortunately it seems that GarageBand seems to want to use it's default locale for storing all it's loops no matter what you do. You can add to the Garage Band library by simply dragging a folder full of loops onto the loop browser and it will add them to the library. Unfortunately when I did this with my 4GB of Soundtrack loops it COPIED them over and indexed them flawlessly but I then had 2 copies of my Soundtrack loops for a whopping 8GB!
The easy solution is to do what I did above but because Soundtrack is a Pro app it lets you manage all your loop libraries and you can just index the default GarageBand locale and delete your original Sountrack loops after they are copied over. You can find the default GarageBand loops in Library/Application Support/GarageBand. One tip for Soundtrack users is to reindex only the folder Library/Application Support/GarageBand/Apple Loops. If you do the whole Garage Band folder you will be stuck with over 1000 instrument sample files as well which will only muck up your index with simple 1 note aiff's that I find little use to and have no data associated with them.
Now I have over 5000 loops available to both GarageBand and Soundtrack
-Jerry C.