agreed. adding sugar mid fermentation is traditional in these big belgian beers. I do it with both my quad and tripel.
Just make a quick syrup from the sugar (1lb of sugar to 1 cup of water is pretty standard I think). Cool, and add once fermentation is past 50% (aka, shows the slightest signs of slowing down).
Primary 1:Citrus-hibiscus-rhubarb Tripel
Primary 2: Rum Barrel Quad
Up next: Rum Barrel Samoa Porter, Rye Saison, a Wit of some kind, summer stuff.