Question on ipchains

I have an internal box being used as a router. I need to route an IP to a
different IP and I am not sure how to do this with ipchains. Here is what I
am trying to do:

10.0.0.253    +--------+  192.168.0.200
------------->| Router |----------------->
              +--------+

The firewall converts the packets into an internal IP which is part of a
DMZ. The router handles the traffic from the DMZ to the internal network. I
need to get a path from the firewall to a server on the internal network. I
have the firewall converting packets into an address that is on the same
network as the DMZ, but the address does not exist. I want to have the
router convert this non-existing address into an address of an existing
server on the internal network. The route is a debian box running ipchains.

How do I set the ipchains rules to convert the IP from one to another?

David