Changing a players password

Hi,

I'm wondering how one goes about changing a player’s password with a PBEM game.
If I use the host password I can reassign one of the AI's to a human player, but that player also gets assigned the host's password. So naturally I'd prefer to change their password to something else.

But I am unable to find a means in which this can be done in the stock game.

Any help would be greatly appreciated.