I don't think what you propose would work well. Stepper drives use a 2 wire input (well, three if you count the return). This gives the drive a step and direction signal from the computer, the 4 wire output to the motor is something totally different.
In order to do what you want, you would have to access the step and direction signal input to the existing drives. The step and direction signals to most drives is 5 volts.