In principle, something like this would be possible, but I personally wouldn't go this route as you'd be double-NATing in every instance. In this scenario, you're treating the firewall like an upstream router, which it really isn't. Assuming your clients are all located locally, you'd still be better off having their own networks directly connected to separate NICs (or virtual NICs) on your PFS and routing them out on their own separately assigned external IPs through the firewall. Otherwise, if they decide to use their own routers, assign them their own external IPs and connect them directly through your pipe to your upstream ISP router. This is just my own opinion, of course.