I believe this is possible.   if I understand it correctly ; the network 
currently sits on a private 192.168.x.x or 10.10.x.x?     if so, uncheck 
"Block private networks" under the WAN Interface setting.    you may 
also need to add a static route.  I have a m0n0 box at a remote location 
that has a wan IP of 192.168.254.2, so I added a static route on the 
other end for the LAN interface for the network 192.168.254.0/24 and the 
gateway of the live IP.  

hope this is what you're looking for.

-cameron