Linksys WRT54G / Setting Up 2 Networks On One WAN Address?

In my home, I have broadband internet connection, which is Time Warner Cable. I also have an Apple Airport Extreme that shares my internet throughout all of my computers. My current set up runs perfectly but the problem is, I have other devices in my home that require a WEP security key. My Airport Extreme is running on WPA but I don't want to lower it's security to WEP just for those devices. What I'm trying to do is create another separate network which has a WEP security key (Linksys WRT54G). So ultimately, I would like to set up two networks, one is the Airport Extreme and the other Linksys, which both run under the same WAN address provided by Time Warner Cable.