Just a little clarification on the above:
mframe=172.18.1.3
peer internal=10.1.1.0/24
our PIX internal=10.1.2.0/24
subnet I wish to NAT peer internal to=10.1.85.0/24
The mainframe and internal routers are already configured to access 10.1.85.0/24. The site to site VPN is currently working, but the mainframe cannot get back to the peer. This is why I would like to NAT their (the peer) traffic to the 10.1.85.0 subnet. The vpn clients, who obtain their IP addresses from a pool consisting of 10.1.84.0/24 IP addresses, are able to access the mainframe. The 10.1.85.0 subnet was added to the routers and the mainframe, but never utilized. Any help is greatly appreciated.