Will I able to share my internet connection using my MBP's airport if I am connected to the internet through an external USB Wireless adapter? I know that I can share my internet connection with other devices using my MacBook Pro's Airport as long as I'm using ethernet (wired) for my connection to the internet. But if I'm using the airport for the internet connection itself, that option to share it goes away.  I'm considering getting a high-powered Hawking USB wireless adapter to use at my cabin in order to be able to get a wireless signal from my uncle next door (with permission, of course) since a built-in wireless card isn't quite powerful enough to pick it up.   

Will I able to then share my internet connection with other wireless devices using my MacBook's Airport similarly to sharing a wired internet connection, considering that I'm not using the airport for the connection itself, but am using the USB adapter?
Yes - Macs have internet sharing where you can use one device to get internet and share it from another.

Most people in a cabin situation set up an AirPort to let all the devices talk and then you would share from the USB to the physical ethernet port. The router would get to the internet from the ethernet and everyone can join the WiFi and share whether or not the USB modem is dialed.

You would set the service order to have the USB modem be the highest connection in the network pane.

What you ask is also possible - just share from USB to WiFi and the Mac will create a wireless network the other devices can join.