I personally wouldn't add sugar to a BW. I typically add candy sugar to my quads, etc... but I figure "barleywine" should be barley. You can control attenuation and balance other ways such as hop schedule, mash temperature, mash duration, mash thickness, yeast pitching rate, pre-fermentation oxygen addition, servomyces yeast nutrient, and fermentation temperature control. I've done, and use all of these techniques to obtain high attenuation regardless of the OG.
If you are making an American BW, you will need a littel residual sweetness to balance the extra hops added. If you are making an English BW, you will be using a lot of crystal and specialty malts, so you can do your best at dropping the FG.
You certainly can add sugar to help drop the FG, but I personally think its cheating yourself out of some extra barley flavor.