I'm having a problem with an ipsec tunnel.

Here is the situation:
My internal single IP for the tunnel is: 172.20.1.11.
My remote endpoint is 156.30.21.200.
(I'm only tunneling one IP on each network to each other)

Unfortunately, the remote network I'm connecting to has a lot of VPN  
tunnels and they already have a tunnel to another network with an  
internal scheme of 172.20.1.x.

Essentially, I need to establish the tunnel from one IP on my  
network, to one IP on the remote network.  Since they already have a  
tunnel with someone else using my internal IP, I need to NAT  
(essentially present it on their network as a different IP) my  
internal IP over the tunnel to their network.

They have specified the NAT address I need to use, which is:
172.20.1.11 Needs to be Nat'ed to 10.0.200.129

Anyone have ideas if this is possible with M0n0wall?

Thanks,

Jimmy