I've used a phone to connect the wifi then share the connection with my desktop through usb before. But I don't remember exactly how I did it, I have a iphone and I had to use a jailbreak tether app cause the stock iphone tether doesn't support tethering wifi -> usb.
A good search term is "Repeat wifi signal with desktop/phone"
A $10 USB Wifi dongle will be something that's nice to have around though even after your done with it