Hi,
I believe this is something that you can do. You would need to give your new router an ip address on your main network like any other device, and then just set a new ip address on a different subnet inside the router for the new network. For instance, say your main network is 192.168.1.xx. Let your router have an ip address on that one, like 192.168.1.100 for example. Then configure the internal ip address for the your new subnetwork on the new router to something like 192.168.10.1 so that your router will act as the gate way for the new 192.168.10.xx network. Its something we talked about in class but I have never had the chance to try it out though, but it should work :P