Creating a wifi hotspot on OSX with a connected USB wifi adapter instead of airport (OSX Lion)
I think this will only work on wifi adaptors that support bridged mode wireless. Not all of them do. The internal Airport does which is why it can be turned into a hotspot. 

I think you'll be able to go this way

Share your connection from : [usb wifi adaptor] 
To Computers using : [airport] 


Setting up all security features as per normal.