Amplifying AC Current for Stepper Motor
I think the practical answer to your question is that you can't put anything between the controller and the stepper motor.

Most 3d printers, and probably yours too use a dedicated stepper driver for each motor. These can microstep and regulate current. It's not a simple on/off sort of signal. If you put some device between the controller and motor then it's probably not going to work correctly.

You may be able to get a new motor that has windings such that that it generates more torque at same current as your old motor. You may also be able to increase the current setting of the driver by adjusting a pot on the board (if you're lucky), or by changing some surface mount resistors.

If none of that works, then I would try to find the step and direction signals, and simply replace the motor AND driver with something more powerful.