Firewall/NAT

I'm having trouble setting up my connection for BattleNet
I have multiple computers in my office.
I have successfully created the rules to enable each computer to create/host the game.
But apparently there is one problem. Each time a computer in my LAN create/host a game, the other computers in my LAN cannot join the game.

Before switching to m0n0wall, I don't have any problem such as above using Linux server with iptables routing.
I used these settings:

iptables -t nat -A POSTROUTING -o LAN -s 192.168.1.0/24 -d 192.168.1.0/24 -j SNAT --to-address=x.x.x.x (my WAN IP)

My question is, how do i setup my m0n0wall to use above command? I've searched all the features in m0n0wall NAT setting. But i don't really understand how to do this.

Thank you, and excuse my poor English.

m0n0wall is FreeBSD, not Linux, and doesn't use iptables. You're not likely to find many iptables experts here, if you explain what that rule accomplishes maybe we can help.