Modding

A newbie question from me.
I understand how to create a new hero, but how do I add this newly created hero to the selection window when creating maps?
Is this specified in the map file itself?
So, do I need to edit all the original maps in order to add one new hero to the selection window? Or is there an easier way?

If by "selection window" you mean the menu for choosing a town and hero, then these heroes are recorded here:
Let's say the town is Haven:


And yes, if the map itself has a restriction on which heroes are available, the hero will not be visible in the selection menu. You will either need to add the option to choose this hero in the map itself, or remove all the restrictions altogether by disabling the function: