The simplest way to get there is to set up your router/firewall to forward the port to a specific computer. Say you open up 10.10.10.10:1028 for the purpose, then forward incoming packets bound for port 1028 to computer ABC in the internal network. It's the same way you would run a web/mail/ftp server on an internal network. Just make sure you cover the security bases on the open port or you could be in trouble.
I guess I just ran all around your question. The router and/or firewall listens on the ports designated by its configuration and forwards the packets on that port to where it is told to do so.
Here's a pretty good article on PAT and NAT from Cisco.
How NAT Works - Cisco Systems - http://www.cisco.com/en/US/tech/tk648/tk361/technologies_tech_note09186a0080094831.shtml