Careful with dog food - the large chunks can choke them to death. And if the seeds have awns (hairs) they can get stuck in their throats. I speak from experience reported here on the Duck Forum.
It may be that the ducks are not very cold hardy, and if you are okay with letting them stay in a warmer environment than an unheated outdoor shelter, then go with that. I have Runners - similar in size to Khaki Campbell's, and a few of my flock do not thrive below 35F. So I adapted a shelter in the walkout basement. Life is good!
Regarding food. Hmmm. What were they raised on? You may need to make a slower shift. I would say it is best to use a well balanced diet for ducks. The most straightforward would be poultry or waterfowl feed, supplemented with some fresh greens. I also supplement in winter with a little bit of good quality cat kibble. But that is a sprinkling on top of their food. I moisten their food, some people on this forum think that is not a great idea due to a concern about mold, but at the rate my ducks eat, that is not a problem.

