Adding sugar will jack up the alcohol to your desired level, but really will NOT reduce the FG at all. It's not a bad idea, but it just doesn't work quite the way you might want it to.
Now.... if you were still in the mode of designing the recipe, you could swap out some of the malt for sugar, and THAT would reduce the FG compared to a 100% malt beer. But, that's not what you are dealing with here, if it's already in the fermenter. Just an idea to keep in mind for next time.