Any way to attach another mesh to another ( Not CTRL + J) Basically, I'm creating a uniform, I want the uniform to have some pockets and the like

I want to create nearly the same as in the picture, but I'm struggling at creating the pockets.


Imagine the cube is a pocket, how do I properly attach it and deform it so it perfectly attaches/merges to the jacket? It's hard to phrase, I hope you understand.
You could try using the Boolean modifier.  Select your jacket, add the modifier, change the mode to Union, and set the Object to the pocket cube.  Hit apply and it should make it one manifold mesh.