I have never heard of using sugar to feed a sourdough starter, but I have used barley malt syrup in bread and it has been great. I keep my starter in the fridge, and feed it the day before I plan to use it with 1 cup flour and 1/2 cup water. If it has been more than two weeks since I last used it I will feed it twice before using it. I have two wild yeast starters that I have had going for at least three years, one wheat and one rye. I also used to have a whole wheat starter, but since just use the other two. I feed the wheat with bread or all purpose flour and the rye with rye flour. I rarely ever add sugar to my bread, only things like sweet rolls, milk rolls, or challah or brioche. Just about every bread I make is flour, yeast, salt, and water.
I wonder how using sugar would perhaps change the flavor profile? Maybe I will split my starter next time and see what using sugar does instead of flour. Might be interesting.