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So we know its possible to have special classes like jet troopers available right when you join the server. But is it possible to do the same with heroes?

EX: If heroes are enabled, it will ask you if you wish to play as one once you hit the unlock value. If you hit yes then you will see the added class of a jedi to your class selection menu.

So if it is possible, how do you change it to already have heroes available to everyone once they join the server?

No, not without changing the actual map's script. Heroes as they are are only available to one player per team (be that AI or human) and have a different type of unlock condition than normal units.