You could use Laser, Pump and Valve outputs right away. In both FPGA and RAMPS they are wired to 12V outputs.
There a relay comes with the FPGA board that manages laser, so it may be able to handle solenoid (depends on your solenoid characteristics) without a transistor provided you do connect the diode line in the diagram above for when solenoid is de-energized (reverse current due to inductance). The pump and valve drivers may not be able to handle a solenoid because of the significant current spike when solenoid is energized (same inductance resists). Though valve in dobot (attached to the pump) is in fact a solenoid, just no idea about its characteristics).
If that doesn't suit you, you could extend open-dobot to wire any other pin from those listed in "calibrationPins" (board-dependent). Take laser/pump/valve as an example.
But, apart from the details, it doesn't seem like a good idea to use solenoids for that application. They are not gentle and literally shoots the plunger as hard as it can. Is there a problem with just telling dobot to press the button (move in Z axis)?
"Is there a problem with just telling dobot to press the button (move in Z axis)?" I need to push smarthphone's volume - , + and the power button which are on the right side so I must move in X-Y axis. But dobot doesn't have enough force to do it.