You have 2 different options.
First, as said before, you can configure the express as a normal client (like the mini) to join the existing network of the extreme. Very simple to set up.
Second you could build a WDS (wireless distribution network). This will create a single network out of the extreme and the express together. Express will not work as a simple client, in will extend the range of network. Usefull for big flats or difficult circumstances (i.e. lots of concrete walls). This way you could walk around with a portable device (macbook or iphone) and connect to the web more far away from the extreme (which I assume is near your dsl outlet). A little bit more difficult to set up, but considering you are using apple-only equipment, this should also work just fine.
Decide yourself, which mode suites you best.