Ah... yes, you can't use the express to extend the range (as Apple puts it), but you can use it as a wireless client on your existing network. The manual's unclear on how to do this, but it does appear to be possible. There's also something in the FAQ saying that you can use it as an ethernet to wifi bridge, and here's the article on how to do it.

I am the Doctor, and THIS... is my SPOON!