When I add sugar to my wild plum port, I pull off two cups of the wine and stir in the measured sugar for 5 minutes. I let what didn't dissolve settle out for 2 or 3 minutes, and pour the syrup into the carboy, leaving undissolved sugar in the measuring cup. Stir carboy gently, repeat with another two cups of wine to dissolve the rest of the sugar, and I haven't diluted my wine with water. It seemed like the sensible thing to do, since I feed it three times during its time in the carboy, and the point is to increase the alcohol level so I don't need to fortify it with store bought spirits. Sometimes I need to warm the wine up to 80 degrees or so to get it to dissolve, but I don't warm it much above that.