Ducks don't really need especial care taken of them during the winter. They've got an inch-thick down coat and way more feathers than a chicken has. They are more vulnerable to predators during the winter, both because the foxes have little else to hunt, and also because they can't just slip into the pond. I don't know your setup, but after about mid-October, I start herding mine inside every night.
I feed high protein meatbird feed mixed with uncracked corn (I've always assumed that cracked corn loses nutrients faster) for a total of 15.5% protein and 4.1% fat during the winter. I haven't noticed problems, and my eggs have good hatch rates (which is the standard by which I measure most feeds.) I do supplement niacin because neither feed is high in it.
You can also feed allflock. Just make sure it matches the minimums provided here:
https://www.metzerfarms.com/NutritionalRequirements.cfm
My temps reached a low of -17℉ without windchill this past winter.