NAT and ISA Server 2004

Hi all. I've here really weird configuration I must deal with. The thing is that I must quick replace a Checkpoint/NOKIA device which is actually dying (I had cars which sounds less than this hard disk man!) and my only viable solution now (believe me, don't make me explain) is to replace it for an ISA Server while I found another solution.

That checkpoint has three NIC, one (internal) with private non enrutable network (192.168.x.x), another one (public DMZ) with two (yes two) public netwroks and the third (external) connected to the internet router,and non enrutable address (10.0.0.x). The weird thing is that this router DOES NOT TRANSLATE the addresses so it's firewall duty to do de NAT. What is the nokia doing via checkpoint nat rules is to translate the source addres of the internal packets allowed to go to the internet with one of the public addresses of the DMZ NIC. It's weird but working.

I've installed an ISA Server, and when I configure the relation between the internal and external network as NAT (instead of route) what I obtain is packets with the source ip translated from an internal non enroutalble address (192.168.x.x) to another (10.0.0.x). I dont' know how to manually specify the source address which I like the packets going out to be "Natted".

I would really appreciate any help on that one.

Thank you!