It can be offered sparingly alongside their sole feed. Offering too much at once will cause an imbalance of nutrients in the bird's diet. There should also be grit on the side when offering anything other than feed to help in the digestion of the matter.
If you are raising light/medium weight ducks breeds, the niacin content in Purina flock raiser will suffice, but if you are raising heavyweight breeds, they have higher niacin requirements therefore will need extra niacin supplementation regardless if they eat duckweed or not.