If you add a USB WiFi adapter can you network share it over WiFi? I've connected to a wired network before and shared that over WiFi. I'm curious if I got a USB WiFi adapter if I could do the same thing with WiFi input. So the USB WiFi adapter would be the network / internet input, and the built in WiFi adapter would be the output.
Yes.  You can share any network device to any other network device using the Internet Sharing settings in System Preferences.