I have not done this though I have thought about it a couple of times.
What I would do is get a WAP, not a card, and config it as a node on the existing network
you might even want to set up a second sme server/gateway and put the wap on a separate network, for security purposes, that way it is not connected directly to your home network with your wired machines (in case or wardrivers)
well, that my 2 cents
steve