Purple (non-Firewalla) USB WiFi Dongle

I am using the Purple and need to run a WiFi antenna to the window to get networking access and the FW has to be close to the work area and not near the window itself.  (That's the required use case, please don't tell me that doesn't need to be done, or some alternative. Lots of business reasons this has to be done this way.)

Can I just use a high-gain USB WiFi antenna?

If I can, and if it just has to be the FW branded one is that hard and fast?

If not, I don't understand why on the cute little Linux box it isn't possible to get another WiFi interface going (although I suspect it might have to do with bridging the interfaces).