HELP Setting up nat with FREEBSD 5.2.1 but keeping mac

[MASSON D.]

Hello,

I need to connect two networks together.
Those networks have total different IP ranges.
Each machine in network A needs to connect with a unique "natted" ip 
adress to the network B, but it needs to keep its own MAC adress.
I try to use FREEBSD 5.2.1 with  NAT to do the job but this sends the 
mac adress from the router to de destination host, wich in my case is 
not allowed.

Can this be done (using netgraph maybe) ?

Massonda from a small country with great chocolates (BELGIUM)
Sorry for my english it's a little bit wrong typed, I know.....