Well, I have heard of something very strange. Most all the Metal Hollow core exterior doors have wooden frames. This is really besides the point.
Hinge adjustment is the answer I belive in your case.
many time if you put a piece of wood the size of a tounge depressor behind the hinge this will do the trick.
First of all you have to think about what a shim does to the door when installed behind either the top of bottom hinge.
Here is what the door does when a shim is added behind the top hinge. The top of the door will extend out more and the bottom of the door will come in.
A shim at the bottom hinge will cause the bottom of the door to come out and the top to come in.
Now, you might have worn out holes that the hinge screws go in. But some wooden golf tees and stick them into the holes and hammer them in lightly. Then saw them off flush with the frame. Now drill a small pilot hole dead center in the new tee with the hinge on to make sure the hinge does not shift.
I do not know of anyone that will and does cut down to size metal hollow core doors. I would think that this would defeat the entire purpose of the steel hollow insulated door.
Also, the frame might not be square in the home's hole. Put a square at the top corners of the frame and if not square, then this is your problem and this is very very very hard to fix once the frame is nailed and secured in the hole.