The problem is, if you intend to bottle and carbonate it, you need the yeast to be viable. But viable yeast will ferment any sugar you add to backsweeten it. At this point, you are kind of limited in your options.
You can sweeten it with non-fermentable sugar, including lactose, stevia, or any of the artificial sweeteners.
You can sweeten it with something fermentable, but refrigerate all of the bottles from the time they have as much carbonation as you like until they are all gone. If the yeast you used as any kind of cold tolerance, you don't want to do this.