I know of no commercial device that does what you describe.
Beamex manufactures process instrument calibrators. One of the functions of those calibrators is a 'simulate resistance' function, used to calibrate temperature transmitters with RTD inputs.
Here is the statement from Beamex on their resistance simulation function for its MC-5 calibrator:
"The instrument under test generates the current for the resistance measurement. The (model number) controls the voltage across its terminals so that the resistance (voltage to current ratio) corresponds to the simulated temperature or resistance. The (model number) can simulate resistance between 1 to 4000 ohms."
This concept works for field instruments that generate a constant, stable current for an RTD input. Not all current sources are the same, but they're generally in the 1mA range. It isn't clear what magnitude of current is used with the variable resistance in your application.
Are you saying that your digital positioner wants to be connected to a 0-5K resistance that represents the position indicated by the magnetoresistive sensor?
The brute force approach is to create a true simulation of a variable resistor, like a motorized pot as suggested by Bob Peterson 5 months ago. Or maybe a microcontroller controlled digital pot where the microcontroller reads the analog output and commands the digital pot accordingly.
But perhaps there is a simpler way. You say the digital positioner accepts a 0-5K Ohms input. But the digital positioner must query that 0-5K resistor somehow. The most obvious way is for it to direct a constant current into that resistor and measure the resulting voltage. But it may also be using some non-obvious way, like using the 0-5K resistor as part of an RC oscillator and measuring the frequency. If you can find out how it is being done, you may be able to simulate the resistor by merely injecting a voltage (which Bob Peterson also suggested), or a frequency, into the appropriate input.
My question is, have you really been wrestling with this problem since last August?