Doors to uninsulated walk-in spaces
To insulate a hollow door - I guess you could try to fill it with insulating material.  You might be able to drill holes in the attic side and fill it with expanding foam.  You might also just make a mess and wreck the door.  Depending how it's made and how patient you are, you might be able to fill it with cellulose insulation.
I have seen attic doors insulated by affixing insulation to the attic side of the door.  One place that was insulated a long time ago, they basically built a 3&quot; deep box out of Masonite or hardy board and filled it with cellulose insulation.  Another I think they just glued a thick sheet of foam insulation to the door.
The old one, the attic door opened into the stairwell but there was enough room between the stairwell wall and the back of the door that the box of insulation didn't interfere with opening the door.  The newer one, if I remember right, the attic door opened into the 2nd floor hallway so interference was not an issue.
If you had an interference issue, you might be able to fix something that's not solid like a bat of fiberglass or rock wool, that would compress when you opened the door and hopefully spring back when you shut the door.
And of course in any case you can't go wrong weatherstripping the door to eliminate drafts.