Yes, this is possible. You will have to coordinate with the services provider to get more information on how the second WAN link is setup and how they want to connect to the 1700: PPPoE, Static IP, etc. This will also help with load balancing your WAN by combining both sources to one device.
I think a better implementation is to make the LAN switch the gateway for the PCs then set a default route in the LAN switch to point all external traffic to the router. This can be disussed furthur if needed.