You mash grains to permit the enzymes to convert starch to sugar. You run off and then sparge to wash those sugars out of the grains and into the wort. Adding sugar should boost the SG, but adding it to the mash will inevitably result in some of the added sugars getting stuck in the grains. Add it to the brew kettle. Unless corn sugar is specifically called for, you are better off boosting the brew kettle gravity IMHO by adding malt extract instead. I usually do a SG calculation at the start of the boil to predict my post boil gravity and add DME if needed.