Modifying a thermostatic radiator valve for micro control
For those not familiar with this device, it is a motorised hot water central heating radiator (thermostatic) valve or TRV. Many brands are available for retrofit with options for wireless control and integration into a home automation system. 

I suspect that an internal limit switch shuts off the motor at end of stroke. The valves are supplied with adaptors to adjust the actuator position relative to the valve pin and this may be good enough to prevent the motor continuously driving against the pin.

Bear in mind that the control of the valve is not drive to a position based on temperature but rather open or close based on difference between setpoint and actual. It's similar to the way you press the accelerator on a car - it's not a particular position for a set speed but rather you act as the feedback loop giving more throttle if under-speed and less if over-speed with the amount proportional to the error.

I'm curious about the innards myself. I'll see what I can find.

[Update]

The device I had read most about is the HomeMatic eQ-3. This is a wireless, battery powered control valve actuator complete with LCD, etc. As in your device this has a motorised actuator to drive the valve pin.



The Installation and Operating Manual. Page 34 states


  After inserting the batteries, all segments on the  display (A) are
  actuated and then the software  version number appears on the display
  for a short  time. Then the valve drive moves the control pin (E) 
  completely back to make installation easier. During  this time the
  display shows „A1“. If the control  pin (E) completely back the
  display shows „A2“.


If the device doesn't have a limit switch it either works by running back for a fixed time (unlikely) or it senses the increase in current when the motor stalls.

Pate 35 goes on to say


  Put the valve drive on the heater valve ... Press the channel button on the valve
  drive one time  briefly. The display shows "A3". The drive closes the 
  valve completely. If the valve is completely closed,  the device
  display shows "0%".


This seems to be the calibration cycle. As you have figured out already you need to know where the actuator first engages with the valve pin and where end of travel is. My guess is that it times its run from A2 to A3, possibly picking up the change in current when it first touches the pin due to the valve spring resistance and then continues on until it detects the hard stop.

I suggest you try hooking up your multimeter in amps mode in series with the battery or motor connections and do some tests. When you connect up the batteries it should go into calibration mode. Note the current at the various positions. (Push a stick or drill-bit into the actuator orifice so you can see its position move from the outside.) If you connect in series with the motor you should see the sign change as the motor changes direction.

Finally, to answer your question, I don't think you can or need to convert the device to a servo. In itself it's an open-loop actuator but when paired with a thermostat it can form part of a closed loop temperature control system.



See also openTRV.