Cisco :: SA520 - Simulate Old DMZ Function On Newer Router?

I have run into a problem replacing an old Linksys BEFVP41 with a SA520. The BEFVP41 had an address on the LAN defined as a DMZ. That address was another router. It is in the same subnet as the local LAN and I am not sure what ports it uses. It is controlled by an outside group and requires much delay and paperwork to change the address. The new SA520 will only support a DMZ if it is in a different subnet than the LAN. Any way to simulate the old DMZ function on the newer router? I have not yet been able to obtain a list of all ports that should go to the second router. Didn't know if there was a way to forward all ports like the old BEFVP41 does by setting a DMZ address.