Current questions and answers regarding the map editor.

To create a neutral hero, you need to place a regular hero on the map with any owner. Then, in the scripts, write:
SetObjectOwner(hero, owner)
where hero is the script name of the hero,
and owner is the ID of a player who is NOT PARTICIPATING IN THE GAME.
Therefore, if all 8 players are participating in the game, it will not be possible to place a neutral hero.
The neutral hero will stay in place, so there is no need to disable MP for it.