I couldn't find any source listing the niacin content of Dumor Chick Starter/Grower 20% either, but, like *most* chick starters, the niacin content will likely be around the 35ppm mark, which is only near about half of what you want for heavyweight breeds (such as Silver Appleyards). Whether supplementation of BY (Brewers yeast) will work or not, depends greatly on the brand.
If you do a quick search online, some brands only contain 5mg per tablespoon, or up to 25mg per tablespoon. If you do your research and find one that's high in niacin, you should be fine, or you could purchase NY(nutritional yeast) which is significantly higher in B vitamins the BY, and is more palatable for ducklings. As a third option, you could purchase a niacin powder, or capsules, and add some to either their feed or water.
Just note, that the pure niacin powders you find online contain a large quantity of niacin, and should be used to the correct dosage, as excess amounts have been known to have a reverse effect and cause structural damage in ducklings.