siro
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SetObjectOwner('scripted_hero_name', player_number_not_participating_in_the_game)
This trick won't work on a map for 8 players.
So, is it impossible to do this on a map for 8 players? I've just encountered mods where neutral heroes exist, but not as objects on the map. They appeared when engaging in combat with any stack of neutral creatures.
To be more precise, here is that mod:
http://heroescommunity.com/viewthread.php3?TID=41303