I attended a presentation on doing exactly that and from that presentation I understood that unless you are licensed to do so, WiFi equipment is not to be modified or used in any way to increase power, range, etc. That is pretty much the case for any device or equipment sold to consumers or commercially here in the US.
Licensed amateur radio operators are allowed, within the limits of their license and only on the WiFi frequencies assigned to amateurs, use antennas, amps, etc. I was quite impressed that with a little bit of effort some rather significant improvements in range could be obtained.