I'm not sure if you are asking about an input to the laser, or something else?
If you have a Chinese laser then no, there is not a pin going to the laser that would directly control power.
Mach3 can output PWM (designed for spindle speed control), this can either be input directly to the laser control signal on the laser board or converted to an analog voltage with something like a digispeed and used to modulate the laser power.
Either way you are bypassing the existing control board for the machine and would need to replace it with a generic 2-axis stepper driver board.
Sorry if I misunderstood the question, please re-phrase it and I'll give it another shot.