No, you cannot fill a hollow core door with putty for a few reasons. The structure won't support the added weight. The interior is honey combed with cardboard for the rigidity, so it's many little compartments, not a big hollow space.I just moved into a new house and while the layout is perfect for my modern-ish goals, lots of the details are very builder grade. In this case, all the doors are you're basic 6 panel composite doors.
I looked into buying modern doors with no panel design (a flat, smooth surface door) but they get prohibitively expensive when they're not hollow.
Is it possible to use some sort of putty or other filler to fill in the panels and not have to replace the door?
How expensive are solid door slabs?