i dont see how that would possibly work unless its a card and everything inside the antenna. you'd also need some way to disable the internal card, etc etc.
I dunno, I ended up buying a Super Cantenna instead, super directional so that worked out perfectly. Problem is, you need to buy a card that will not only be recognized by the Airport software but ALSO allow an external to be plugged in. I ended up opening the powerbook and wiring into the existing card with a new mini coax plug that i wired onto the cantenna.