If you take out all the servo electronics and just connect the motor to the motor shield it should work. I have used small servos as a cheap supply of small motors and gears.
However you can also convert the servo to continuous rotation while leaving its electronics intact and then you control it with the servo library in the usual way except that the "angles" now mean the speed and direction of the servo rather than its angular position.
Awesome, I will try later!
Yep, I was using this way, but one Servo I hacked don't change the direction (and the other one, don't stop when I put "servo.write(90);"). I revised everything, I don't know what I did wrong. So I was searching one solution, and I thought about connecting the DC Motor situated inside the Servo to the DC Motor ports in the Motor Shield.
Thank you, cheers!