Smashed peas are great for duckling, and actually contain a fair amount of niacin, just have grit source available in a separate dish so the ducklings can digest, and absorb the treats better.
I'm coming to the conclusion that you have other ducklings, but there smaller breeds, so you're figuring they don't need the extra supplementation, therefore you're separating them during supplementation time, correct (?)
A little extra niacin won't hurt the others, so if the Pekins get ancy when you separate them, just leave them together when you offer the B complex laced-treats.