AirPort Extreme antenna upgrades
is bossible upgrade the new apple airport 802.11ac. extreme with ax external antenna like with the old one?
Yes, it is possible, however I doubt it will not help. Apple already made an excellent design, not much room to improve
It would be a tricky job to emulate the same type of setup you will need externally. You see there are six (yes six!) antennas you would need to setup and they need to be setup in a fixed matrix so shaping can be done. Otherwise you won't get the full benefit of new 802.11ac services.
Getting to the ports is not very tricky as you can see in Step 9 antenna plate and Step 11 ports on bottom edge of the Extreme teardown.
Frankly I would leave it alone.