You are asking two questions??:
"Is it possible to make a transition car with scale coupler at one end and traditional (hirail) at the other?"
Yes. To make a transition car all you need is a scale (Kadee coupler) on one end and an AF (claw) coupler on the other. No wheels need to be changed.
Your second comment is a little confusing:
"I guess I would have to change the hirail wheels to scale on the front and use a scale truck/wheels/coupler on the back."
You would never mix wheel types (scale, small flange on one car end and hi-rail on the other). If you are converting an AF type car where the trucks and couplers are molded together then yes, one of the trucks would have to be changed to allow the addition of a scale coupler.
The easiest method is to use American Models or SHS/MTH trucks on both ends of the car. The couplers are removable with these trucks and allow the addition of scale couplers after removing the hi-rail coupler from one end. The wheelsets on this type of truck are easily changed to either scale or hi-rail as desired.