You don’t need zone expanders — you can go to Edit Options -> Number of Stations, and just add 8 to whatever number you already have. Even if you don’t have the physical expander, it’s fine. As I said, we are talking about a virtual station.
If you want to add a switch to open master valve directly, you can add a switch between your master valve port and OpenSprinkler’s ground (on the orange terminal block, the pin closer to the USB port is the ground pin). That way when you turn on the switch the master valve should open. Do not use OpenSprinkler in parallel with another sprinkler controller, unless if you have analyzed the internal circuitry of the other controller and make sure it’s compatible with OpenSprinkler (otherwise it might result in damage).