There may be other ways, but I think you have two to choose from
1) use a VPN .. your external client is logged on to you local (internal) network as if they were physically "inside" the firewall.
OR
2) Create a DMZ and set up a SME server in the DMZ. You need a firewall with 3 network cards. One for the Internet connection (RED), one for the local network (GREEN) and one for the DMZ (ORANGE) ...the colours are just a convention used to refer to the 3 networks. With this arrangment, your DMZ server is available from the internet (with no access to the GREEN network) AND it is available from the GREEN network.
Read the Manual for advice on setting up a DMZ and search these forums for HOWTOs on DMZ and VPN.
Good luck
Ian