In my opinion, you are not going to be happy with a cider backsweetened with a non-fermentable. You want to use sugar, but you obviously can't just dump sugar in there, as the yeast will ferment it out.
The easiest (but still a PITA) way to backsweeten in a bottle is to add about 1/2-3/4 of sweet apple juice to the bottle. You then literally have to make an educated guess on when the bottle is carbonated but not over carbonated (about 2-3 days, but very unpredictable), and either pasteurize by heating each bottle to 180 for 2-3 minutes or 140 for 10-15 minutes (sliding scale in between), or put the bottles at fridge temps in perpetuity until they are consumed to hibernate the yeast.
You can also add camden tabs to each one, but that can get a bit pricey and add a bit of a sulfury tinge to the final product.
The way the commercial cider makers do it is they typically add concentrated liquid sugar or sweetened juice to the precarbonated hard cider on the bottling line, cap, then run them through a pastuerization tube (like a counterflow chiller with a tube within a tube, bottle inside, hot water outside) where hot water kills the yeast.
In a keg (and this is how I do it), just add 1/2 a gallon of sweet apple juice to 4.5 gallons of fermented hard cider, and put it in the kegerator at fridge temps. Done.
PITA in bottles. Pretty easy in kegs if you have a refrigerated serving chamber.
