I think what you'll need to do is configure one of your routers to act as the core router for your internal LAN. Then say you used 192.168.100.x address scheme and assigned 192.168.100.1 or something to the routers Ethernet interface. The PIX can still be DMZ firewall with an inside address of 192.168.100.x. You just have to configure the a default route to your internal LAN pointing to 192.168.100.1.
The command would be "route inside 192.168.100.0 255.255.255.0 192.168.100.1" telling the PIX that to get to that network go to the router and the router will handle the routing leaving the PIX to handle the firewall stuff and the NAT'ing.