Although the Pi 3B and 3B+ have on-board WiFi support, we understand there may be instances where you'd like to use a USB WiFi adapter to extend the range of your wireless access point. This should certainly be possible, however you'll need to modify the yaml file that you use.
Here are the steps needed:
1. Generate a yaml file using our wizard here, or open the current one if you already have one.
2. Replace the text "wlan0" with "wlan1"
3. Save the file and copy over to your USB drive.
4. Connect the USB drive to your player, and it should reboot.
Once it comes back online, it should begin using the WiFi adapter.
We recommend that if you need to use a WiFi adapter that you use the official Raspberry Pi USB WiFi Dongle. These seem to be very difficult to get a hold of, so the part that we tested and confirmed with (a Panda Wireless Adapter - Model PAU06) can be found on Amazon here.