How to configure Endian Firewall in order that a network connected behind a router behind green can also be handled?

I assume you are using a network similar to the described one:
network_behind_router --------- router -------------- green_iface_of_efw
10.10.10.0/24          10.10.10.1 || 192.168.0.100          192.168.0.15

To add static routes to green just:

1. go to Network > Routing > Add new rule
2. Type the network behind your router into the "Destination network" textfield (in this case 10.10.10.0/24)
3. Type your router's green IP address into the "Gateway" textfield (here 192.168.0.100).
4. Click on the "Activate" checkbox and then hit the "Save" button.That's it!