The only USB wireless adaptors that work for the xbox are the ones that are made for it. Although, I'm pretty sure the new xboxes have them built-in.
An alternative. I'm not 100% sure on this but I think you can get a second wireless access point/router and bridge it with the first and then connect it to the ethernet port on the xbox.
Of course, you would need to go to the expense of getting a second access point/router and the hassle of setting it up.