Sure, you can add it after fermentation starts. I've never seen it at a grocery store - only homebrew stores. Alcohol reduces its effectiveness, but if you double the recommended rate it should still work.
Some options for backsweetening - xylitol works great if you don't want to kill the yeast, as its non-fermentable. I don't keg, so this is what I do when I want to backsweeten. Be careful with it, a little goes a long way, and some people's digestive systems can't handle it well. If you use sorbate and force carb, plain table sugar works great, although some people use frozen apple juice concentrate. Pull a sample and find the amount you like, and scale up.