Use of a DC motor with encoder to control steering

For my senior design project my group is converting a John Deere lawnmower to RC. We need to control our steering with a DC motor that uses a rotary encoder. I know the Pixhawk has functionality for encoders on wheels of the rover, but I need to be able to track the position of the steering. Ideally I could make the motor function like a servo where it will snap back to center if the control stick is released. Any help on this is appreciated since I have minimal experience on editing the firmware for these things.