I have been looking into this as I am having connection issues with a wifi router I was using as an ac wifi bridge. (Netgear R6250)
There is no wifi USB adapters that work under 10.11. Bear Extender has a USB 2/3 ac wireless adapter that they said would have beta drivers in December.
That leaves something like a wireless bridge. I have been scouring the Internet, almost all of them have both good and bad reviews. I would love just to purchase another AirPort Extreme, but there is no way to set it up in a join only mode (just extend, which halves the wifi speed). You can set an airport express up to join a network, but is is wifi N only.
Not sure what I am going to do myself...