Sure. I actually like to add “flavors” after primary fermentation.
I mix cocoa powder with enough hot water to form a paste and add that to the beer (makes it much easier to get the powder into the liquid and improves the flavor in my experience). How much chocolate flavor do you want? 4-8 oz in 5 gallons is the general range.
You can add orange zest directly, or you can make an extract with vodka (or another spirit if you want added character). Again how much you should add depends on how much flavor you want. A couple oranges is probably about right. You can always add more if you first addition doesn’t give you enough character.
If you have the space/effort you can split the batch if you want a case plain and a case with chocolate/orange.
Hope that helps, welcome to the board.