If you leave this page, the information you have entered will not be saved!
Are you sure you want to leave this page?
In one word, you cannot. You can replace the door but that's not what you asked.
Some European countries insulate doors by installing some leather/vinyl covered insulation resembling the old couches of the 60's in fast food restaurants. It might not be your cup of tee. In some cold areas, they resort to double doors 4" apart, one swinging in, one out.
A storm door can help but not with good looking entry systems.
You can get by with purchasing a new fiberglass door slab only - keep the frame. $250-$400 plus installation.