You can do this in the Extended Networking menu, under Port Security. Just add a new port range configuration for the port that you are wanting to map and enter the IP address of the machine on the LAN under the "redirect packet to IP address" option.
Below where you place the IP Address to Redirect, there is an option called "Don’t Translate Source IP", you may need to have that checked too. In the Case of PPTP connections, it is needed, as shown in the image below.