Hi,
You are going to have a hard time forwarding a port that belongs to the SME Server. You have to kill that port on the SME first, or port forwarding will not work.
However, I don't think you want to port forward at all. You can probably use proxypass to get this done. Do a search and see it it works for you. Basically, it will redirect an URL to a different IP Address on the LAN. I believe it only handle web traffic.
If you need non web traffic, try the port forwarding, or try a contrib called 1-to-1 NAT which will take bind a second IP to the outside interface and forward all traffic to that (new) IP into an IP on your LAN. I'm currently having a little trouble with this on SME7, but it's a cool contrib.
Hope this helps.