Holy hell, that's a needlessly-complicated setup. My first suggestion is to ditch the DMZ setup..completely. It's not necessary and terribly insecure. The second router idea, while it would work, is essentially the same thing you're already doing. I imagine you're using ICS (Internet Connection Sharing) to accomplish this, which means your workstation just became a router and is acting as a gateway for the server. You would have to do your port forwarding there and, last I checked, the interface isn't very intuitive, so expect complications. Your best bet is to throw a switch in front of your workstation. This will give you the added ports you need. Plug the switch into your modem and plug the server and workstation into that switch. Again, disable the DMZ and forward the ports you need to each machine. This is a FAR simpler solution as you only have to manage a single gateway.
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