Is it possible to control a 4 wire Bipolar stepper motor with a PWM signal?
You may need to build your own controller unless you can get the proper drive outputs from your controller.  Generally, stepper motors are driven with step and direction signals as they allow very precise control over which direction the motor is rotating and precisely how far it turns.  If you can't get access to the coil center taps, then you're probably going to have to either find a different motor or roll your own controller.  It would not be very difficult to get an Arduino to read in the PWM pulses and drive a stepper or two with a motor driver shield.