1. Objects > Address book > InterfaceAddresses, add
lan_2_ip - new DFL's IP address (ex, 192.168.2.1)
lan_2_net - new net (ex, 192.168.2.0/24)
2. Interfaces > ARP
Add ARP publish on LAN interface with lan_2_ip
3. Routing > Routing tables > main, add routes
lan_2_ip core 0
lan_2_net lan 0
So, now DFL knows new IP address and network.
4. Objects > Address book > InterfaceAddresses
Add group lan_nets = lan_net + lan_2_net
If you want to allow traffic between your netorks, add rule
Allow lan/lan_nets lan/lan_nets all_services
If you want allow accessing to Internet for new networ, change lan_to_wan rules - replace lan_net in source network to lan_nets.