port rewrite from lan to wan

Hi All

 I want to setup the following rule in monowall.

In my LAN network if any host send anything to a *particular External IP
Address*[* A.B.C.D*] and *port 25 *=> I have to send to the Same IP and
Different port out [* A.B.C.D*] and *port 26*

I could not find any place to make this port rewrite for the internal
network.

Any help is appreciated.

Thanks
Lloyd