Port forwarding isn't that complicated if you're using iptables. Sadly I never mastered it on ipchains (I never had to) — I thought ipchains had more or less died out by now.

Assuming your firewall machine is not the same machine that you want to run the chat software on, I believe the command ipmasqadm portfw is involved and I've found a little bit (http://www.ox.compsoc.org.uk/~steve/portfw-2.2.html) of stuff about it.