PPM/PWM controlled, closed-loop stepper motor?
The whole reason for a stepper motor is to get accurate positioning without a closed loop feedback system. It could be done, but will have a larger deadband and be much slower than a normal servo. However there might be applications where it could be useful to operate a stepper motor this way.


  one way that it could be made is by buying a cheap servo, desoldering
  the small DC motor from the control board, making some kind of
  interface that translates the -5v to 5v DC voltage to stepper pulses,
  and outputting that to a stepper motor.


This won't work properly because the servo control board is tuned to the characteristics of the motor and gear train. 

The simplest design with fewest parts would probably be a small MCU and stepper motor driver IC. The MCU would receive the PWM signal and measure its width with a timer, operate the stepper motor phases via the driver, and read the pot with an A/D converter. Modules and source code to do all these things are readily available (would just take a bit of programming to get it all working together).