That would not be very hard.
All you have to do is read the pot values and scale them to be in the range of the MIDI parameters used by GarageBand.
Then send it over to the PC as MIDI.
There are many examples around sending MIDI from Arduino. One good source of information for this is here:
little-scale: arduino (scroll down a bit)
EDIT: Implementing preassure / velocity sensitive pads would be quite a challenge though.
MikMo