5ghz and 2.4ghz

Ok, here is my problem.
I have an Airport extreme running at 5ghz in my upstairs office with extend this network enabled, Downstairs i have an Airport Express connected wirelessly to the extreme in Extend this network mode. All works great with my macs. But, my iphone and the wifes netbook cannot connect because they only use 2.4ghz. Can i connect a spare wireless router i have to the ethernet port of the express and create a 2.4ghz network for them. If so, what settings do i use for DHCP etc on the router.
Hope someone can help.

Regards

Paul

Didn't realize it was such a difficult problem to resolve? Anyhow, i have managed to resolve it myself. One of my old routers had a use as access point only setting, and with this on and plugged into the express it does exactly what i want. No settings required apart from assigning the router an ip address in the same range as the extreme. I now have a 5ghz network for my macbook and imac and a 2.4ghz for my iphone and the wife's netbook. All are running on the same network and can see each other. In case anyone is wondering why i have set it up like this, it is because i am getting very bad interference from somewhere that will not allow the macs to connect to a 2.4ghz network. Still need to find the source.

Paul