Adding custom heroes without replacing the default ones.

I want to add my own hero with his PERSONAL skill (taken, for example, from another faction – let's say, adding the Guardian Angel ability from Humans to a Union hero). Can I do this without replacing the standard heroes?

Yes, no problem.
1) Where should it be added? Only as a mod for a specific map or for all maps?
You will have to modify each map where he can appear.
2) The easiest way is to download and analyze this mod. It will be more difficult to analyze NHF. In this mod, all standard maps have already been reworked.
3) You can add the Guardian Angel to any hero, but it will only work if the hero has at least basic Light skills.