If you want to use a public IP address other than the WAN interface address with Inbound NAT, you need to define the address in Server NAT first.
Click Firewall -> NAT, and click the Server NAT tab. Click the to add a new entry.
After double checking your entry, click Save and Apply changes.
The first time you add a Server NAT entry, you may have to reboot for the change to take effect. If you are prompted to reboot, you must do so before you can use the Server NAT entry.
Now if you go to the Inbound NAT tab and click the to add a new entry, and click in the External address box, you will see the Server NAT entry you entered above.
Depending on the way your WAN connection is setup, you may also need Proxy ARP for Server NAT to function.
If any of the following applies to your setup, you should be fine without proxy ARP:
the additional IP addresses that you're trying to use are part of a subnet that is routed to you by your ISP (i.e. your ISP has a static route for that subnet with your m0n0wall's WAN IP address as the gateway)
you're using PPPoE or PPTP on WAN
Using proxy ARP under these conditions will not achieve anything. If however you use static IP addresses or DHCP on WAN and don't have a routed subnet, adding proxy ARP entries for the additional addresses/ranges/subnets in the webGUI will make sure that m0n0wall responds to ARP queries for these addresses on the WAN interface.