If you haven't already purchased the Extreme, then I would suggest going with a router that's more full featured (something from Linksys perhaps). You may also need to flash a custom firmware like DD-WRT to enable more features.
If you do already have the Express, then Comeagain?'s suggestion of using an Express (or other wireless to wired bridge) to connect to the Extreme's guest network and share its connection to a wired client.