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.