From what you are describing it sounds as though you need to run the Firewall in transparent mode. The only IP address the firewall will have configured is to allow administration access. Hosts on the trusted and untrusted sides will belong to the same subnet and (access rules permitting) will be visible to each other natively.
Alternatively if the source and destination hosts belong to different subnets, but you want the respective IP addresses to be visible to each other it is a matter of setting the NAT value to "None" in all applicable rules governing traffic to/from these hosts. You must ensure that appropriate routing is in place to ensure the traffic flows correctly.
Hope that helps.