I believe it is possible, despite the arrogant ramblings of ericwong.
You are looking to do something like:
http://wiki.openwrt.org/doc/howto/usb.reverse.tethering
I've not tried it, so I can't help you further, but if you succeed I think many would like to know know about it. The alternative is, of course, to buy an usb ethernet adapter, but a solution that would go from usb to usb directly would be more elegant and save you money.
Good luck!