I think what you are looking for is called a "wireless bridge" [see Edit 2 below]. You can buy specially-designed wireless bridge devices for lots of money ($100 to $200 or more), or you can find an old wireless router you aren't using that's compatible with DD-WRT. (The latter option can be free if you already have an old, compatible router.)
DD-WRT is open-source router firmware that has the nice little feature (which I've made use of quite recently in a situation probably similar to yours) of being able to set the router to act as a wireless bridge. You simply configure the router via a typical web interface to connect to your already-active wireless network, and then any device you wire to the DD-WRT router will be connected to your network.
I would recommend starting from DD-WRT's wiki.
[Edit 1] In case you're looking for a specific router suggestion: When I did this, I used an old Belkin router, but I've heard that the Linksys WRT54GL is an excellent router on which to use DD-WRT, as well as in its own right.
[Edit 2] Per @bbaja42 in the comments, here is an informative page at DD-WRT's wiki explaining some differences between an actual bridge device and a client router.