If you are using m0n0wall and not pfsense, then the best place is to ask on m0n0wall forums and mailing lists. While pfSense was based on m0n0wall, the user interface and feature set has changed. We could get you close and I will attempt that. Since you don't need firewalling, I doubt that you need NAT also. Given that, you can actually turn of firewalling and turn either pfsense or m0n0wall into a router. I don't know where it is in m0n0, but in pfSense, it is in System -> Advanced -> Firewall/NAT. There is an option to disable the firewall. It will not NAT either with the FW off.
As far as routing, you do not need to create routes on subnets assigned to the pfsense or m0n0wall system. It routes between them automatically.So unless you have a subnet behind another router, then you will not have to create a route at all.