Well, true "backsweetening" is when you keep out some of the original must, freeze it or refrigerate it, and when the fermentation is done, the wine is stabilized with sorbate and campden and then the stuff you saved is added. It gives it a fresh fruit flavor and some sweetness.
Most people use the term incorrectly, though, and use the term to mean to sweeten it when fermentation is done. You can do that several ways- stabilize the wine (sort of "kill" the yeast) and add sugar to taste in a syrup solution or add a can of apple juice concentrate. to taste. You can't carbonate if you do that, because you have to stop the yeast from re-fermenting.
You can add splenda to taste and then bottle it carbonated or still. You can just keep it the way it is and carbonate it. You can bottle it the way it is and not carbonate it or sweeten it and serve it with 7-up. If you keg, you would have even more options!
Um, I think that's all the things I can think of right now!