Yes you can!
A little over a year ago, I visited a family in my state that’s much further along in their farming operations than we are. While the father still works a regular job, they have managed to buy a nice chunk of land (80 acres, I believe), and build a house and pole barn on it. When we visited, they had just slaughtered a number of broiler chickens (30ish?) and had another 30(?) egg layers. They also had 10-15 beef cows, and had a rented bull in with their herd when we visited. However, what got my attention was the large white duck walking around the yard.
I asked the father about the duck, and he indicated that his youngest son (about 10 at the time, I believe) had wanted a duck. He said “When a boy says he wants a duck, what do you say?” I didn’t know the answer, so he told me: “What kind of duck do you want?” This resonated with me because I had asked my parents for a duck for my second birthday. I actually told them I wanted “A duck, a grape, and a trailer for my tractor.” I got the grape and the trailer, but only a stuffed toy duck. So when my son started wanting to look at the ducklings every time we went to Tractor Supply, I decided to get a book and learn a bit about ducks. Then we walked in one day and the ducklings were 1/2 off, and my son didn’t want to stop looking at them when it was time to go. So I brought home 2 ducklings.
The ducks lived for a couple weeks in the fenced garden during the day and in a plastic crate in the garage at night. Then came the day for introducing them into the chicken pen. I was a little apprehensive. I had read online that chickens like to pick on ducks, since they have sharp beaks and ducks do not. My ducklings were only about 1/2 the size of the chickens at this point, and did not have hardly any wing development. So I made sure that I had plenty of work to do around the chicken pen so I could keep an eye on things.
I needn’t have worried.
The chickens didn’t pick on the ducks at all, and let them have access to both food and water. The ducks largely ignored the chickens, but followed them into the coop around dusk and slept on the floor. Adding the ducks to the flock made very little more work. I do have to refill the waterer more often, as the ducks drink far more than the chickens, and also wash their bills in the water, tending to muddy it. Still, that’s a minor inconvenience. If you’ve been thinking about adding some ducks to your chicken flock, but have been holding back because you don’t know how the two species will do together, my advice is to go ahead, as it has worked well for us.