Of course you can. That’s pretty much what I did but I only have chickens not ducks.
What kind of floor do you have in it? As wet and messy as ducks are, if you water in there, you might have a problem unless the floor is dirt, but I’ve never had ducks in a coop so I really can’t say from experience. You might want to keep the water outside with ducks.
Something to be aware of is that chickens make a lot of dust. That comes from their dander, the bedding gets broken down into dust particles, and their poop dries. Chickens scratch a lot and turn all that into dust that gets on everything. With that other stuff stored in there, make your walls pretty solid to keep that dust off your furniture or plan on some serious cleaning before you use it. Another option is to cover that furniture and such with a drop cloth.
You may not have as much space to spare in there as you now think once you section some off for a coop, but be a bit generous in laying out the coop space-wise. That’s as much for your benefit as the animals. I’d think of something big enough that I could walk in and still have room to move with the roosts, nests, and all that there. I’d want the human door inside the shed so you can get in and out without standing outside in the rain, trying to unlock a door with your hands full of stuff, but that door will take up room for its swing. For space reasons you may want to have that human door on the outside but don’t put it where rainwater drips on you when it is running off the roof.
Now I’m going to say something that kind of contradicts what I said earlier. In your set-up, sectioning off a section of a shed, you have an opportunity to provide good ventilation for your coop and reduce the problem of rainwater blowing in. This depend on if that shed is well ventilated or pretty air-tight. If it’s air-tight, don’t do this. But if it is well-ventilated, I’d open the top of that interior wall up and cover it with hardware cloth. I’d still want ventilation up high on at least one outside wall under any overhang. Also have a way to get good ventilation down low in the summer that could maybe be closed off in the winter. In really cold weather you don’t want the ventilation to cause a breeze that is hitting them while they are roosting but in summer a breeze feels really good. Heat kills a lot more chickens than cold. It has something to do with them having a permanent down coat.
Another thing that will take up room is that you will probably want to store feed and other supplies outside the coop but in the shed. I use a metal garbage can to keep the rats and mice out. Think about where you will place that when you are figuring out your doors. You’ll probably wind up with some other stuff too as far as supplies for the chickens, maybe oyster shell or something to help you clean the coop. You always wind up needing more room than you think you’ll need.
If you have electricity out there, I’d put a light overhead in the coop so you can see in there in the dark. Put the light switch outside the door so you can turn the light on before you walk in there. I’d also have an outlet where you can plug in some electrical appliance in the coop section. Even if it is inside, use an outside fixture with a cover so you can keep it closed and keep the dust out.
I think you have a great opportunity there to have a really nice coop. It will require cutting holes in it for ventilation, probably a window, and a pop door, but a substantial building is there with a very important roof. That makes it easier. Good luck.
