Unless they're talking about some kind of dark candi sugar, spare yourself the money and use regular table sugar. Sucrose is sucrose. Now to add. Baltic porter is a very big beer and the yeast would prefer the sucrose to the maltose from the malt, adding it early could result in underattenuation. So here's what I'd do. I'd add the sucrose after the yeast has had a chance to chew up some of the maltose. Let the beer ferment a few days, then make up a syrup of about an equal weight of water and sugar, boil it, cool it, then add it. That'll help the yeast eat up the maltose before they get lazy eating the sucrose.