Nutrena All Flock **should** have enough Niacin to support and adult duck of most breeds. Very skinny is not one of the typical symptoms of niacin deficiency.
That said, there is no downside to briefly offering a bit of yeast with the feed to up both the protein and niacin content. Any supermarket yeast will do - simply turn to the back and look to ensure it has included niacin on the nutritional label - from that, you will be able to calculate dosing.
and no, I don't think its niacin deficiency, but while you are dosing, you can keep a close eye on your bird and look for other symptoms that may help dial in a diagnosis. Start with the poops. Bird poops don't lie.