Is there a way to route to more than one subnet on the LAN side of the RF550? We have a VPN set up between two locations, the RF550 at one location and an ISP owned and managed PIX501 firewall at the other.

No, the RF550VPN can only do VPN on the LAN subnet that is part of the LAN port.

We have another product, the RF600VPN which can handle multiple subnets behind the LAN over the VPN tunnels.