A prefab chicken coop would not be good for ducks. The don't do well on ramps. They can learn to use them, but it is best if the incline is shallow.
A modified dog house could definitely work depending on the size and how many ducks you are getting. They need good ventilation to keep things dry inside. Ducks need at least 4 sqft per bird in the coop. Modifying a shed is another good option.
Ducks require water deep enough to dip their head in to keep their eyes and nares clean. And they will make a complete mess of their water. They love making things muddy and digging holes in the mud with their beak. So you will need to think about how best in your area to deal with the inevitable muddiness.