Fortigate as VPN server, only internal interface(s)?

I need to implement a VPN solution on a 10.0.0.0/8 network, with an already functioning router/firewall.  

I'm envisioning plopping a fortigate on the network, and port forwarding (or 1:1 nat) from the existing router to the fortigate, and using it to allow external users to access the internal network.

Before I drop the cash, I'd like to make sure that's a possible and supported configuration.  Can anyone chime in?