What you need is a "wireless bridge" - where you plug a wired ethernet device into a wireless hub/switch which connects wirelessly to your main router.
I last set up something like this 2 years ago. At the time, dedicated wireless bridges were stupidly expensive, but the Linksys WRT54G could be flashed with a replacement firmward (DD-WRT) which would allow it to work as a wireless bridge. The WRT54G has since been redesigned, so I don't know if it's still compatible with DD-WRT. DD-WRT supports a lot of differnt routers, though, so there's a good chance you can get something off the shelf and modify it.
Off-the-shelf bridges may have come down in price since then, too, so that might an option as well.
