I can't really imagine how hollowing out a planet could work, but I suppose if you figured out a way to do it you could replace the iron core with superheated w

I can't really imagine how hollowing out a planet could work, but I suppose if you figured out a way to do it you could replace the iron core with superheated water or something similarly less dense than iron. That's why I also suggested just building a shell from scratch. It would effectively be a very tiny Dyson sphere.