Apple juice concentrate

Do you have to use preservative free concentrate to back sweeten in your kegging your cider?

Just to sweeten it you are fine. If you want to naturally carb it and haven't done it then it might.

No. The whole reason for not using juice with preservatives pre-fermentation is inhibiting yeast growth. If it's already done and you plan on just back-sweetening and keg carbing, using a juice or juice concentrate with preservatives like sorbate is just fine.

Now bottle carbing... That's a different story. I've never dabbled with that beast before (I'm spoiled and bought myself kegging equipment before anything else when I got into brewing) but there are plenty of other members that have it down to a science and would be great help.