It would be easy to fit casters to most cases but it will require drilling unless you are very lucky that the case has holes and these holes line up with the mounts for the castors, make it the first thing done when you get your case BEFORE ANYTHING goes in it and make sure ALL the swarf is removed. You will need to get nuts and bolts as well as the metal will be too thin to either tap or use self tapping screws on, they would hold for a while but will come loose.
Why not just get a case like a CM Stacker that has them already!