NAT rule for network segment

Before I call support, does anyone know if/how to create a NAT rule for a LAN segment on a USG100? Basically, I want everything on LAN2 to NAT to a specific public IP, and then to be able to create port forwarding rules (PAT) beyond that, so for example, all of LAN2 uses 3.3.3.3 as its public, and inbound to 3.3.3.3 on port 25 forwards to a specific host on LAN2.