All questions regarding mapmaking for Heroes of Might and Magic 3: The Shadow of Death.

How do you copy a part of a map? Or generate a separate Dungeon. There is a single-level map. I want to add an underground level to it. Either from another map or just with a random generator. I found some program called "copymap," which is like a console command, but it doesn't work on 64-bit systems. I found something about attaching it to the editor, but only for WoG. But I need it for SoD or HotA.

Answering your question and my previous one, I'll say - the ability to copy a map exists! It's strange that people on this specialized forum couldn't help us with this most important question for mapmakers.
It turns out it's quite simple. This function appeared in the WoG map editor version 3.3, and it allows you to copy the entire landscape, objects, etc., both within the same map and to other maps on an unlimited scale, whether on the surface or underground! In your case, I would install WoG, copy, edit, and save the map (in the specifications, check the box for SoD). Then, reinstall SoD on your PC, and that's it. I haven't tested this method, but it seems like it might work.