Subnet routing with Sonicwall Tz170

I am moving a bracnh office into main office behind the firewall. I will have a free Tz170 which I would like to use to the main office and branch office subnets. I would like to keep the bridge behind the present 2040 firewall. Can the main subnet be connected to the OPT and the branch office subnet be connected to the TZ170 LAN port with both network set to LAN work? Since there is no WAN connection will I have to set the default route to piont to the 2040? is there a better way to do this?