You can in fact add an encoder to any stepper motor, including this one.
First, you will need first to use an encoder with the correct bore (8mm) or machine the rear shaft to the correct size.
Then, you will also need to find a way to attach it to the back of the motor. Holes spacing are pretty much standardized but this motor appear to have no holes in the back. It is possible to drill and thread holes but difficult if you have not done that before. This is risky too since you should never drill too deep, which could damage the motor. I would then suggest to attach the encoder with double side tape or much better with epoxy.
Do a search for
AMT102-V. It is a very good and not expensive encoder that I've used before.This is a 'kit' that also come with a 8mm sleeve that will make it fit the rear shaft directly.
For the 'controller', you can certainly use a combination of mesa card but I will let the experts elaborate about this since there's many possible choices. Have a look here:
mesanet.com
.
