A manual switch is doable. Basically each station port (1 through 8) is internally connected to a triac that functions like a programmable switch. When the triac is turned on, it connects that solenoid wire to the circuit ground. Since the other wire of the solenoid is connected to COM (common), it completes the circuit from 24V AC transformer to solenoid, and the solenoid is turned on.
You can put a switch (in parallel to triac) between the solenoid wire and circuit ground. This way, when you turn on the switch, it activates the solenoid. You can also connect an LED (with a current limiting resistor) in parallel with the solenoid, so that every time the solenoid is turned on the LED also light up.