Gamebird starter is intended to be fed to Gamebirds, I'm not saying it can't be fed to waterfowl, I know some do, but you might want to contact the company and ask for the niacin content per kilo. I tried searching a niacin estimate, but it seems it greatly varies with the brand and species. Gamebird feed has a little higher protein then duck feed, but many feed it with no reported problems.
Ultimately, this will tell you if you need to supplement with niacin at all.
What breed of ducks do you raise?