So i currently have a working circuit that uses an hbridge model L293D to control 2 motors and then have code that uses serial monitor to control two motors using the serial monitor. What i was wondering is if i can add a mosfet to my circuit to control the amount of voltage my hbridge receives in order to control the motor speed as well?
Why not use the PWM on the Arduino to control the L293D?
Usually the controller provides low-power PWM to the motor driver to control speed (by how much of the pulse is ON and how much OFF) while the power for the motor goes straight to the driver. You want to take care how many volts the motor power is to not burn the driver or motor up, but that's where you up the motor voltage not through the control signal.
You may want to get specs on your motor and driver. Be sure to note max voltage and current, it is up to you to stay in limit.