How do I make a normal unit, hero class?

For example, Kel'Thuzad's Necromancer Form, how do I give him a hero flag so he can learn hero abilities and gain level

You can only make him a hero. Or spellbook maybe.

Well how do I make him a Hero?

You start with a hero and change his model, etc.

You can also make a copy of the unit and change the id from u000 to U000
(uppercase first letter).
To do that you'd need a hex editor and edit the war3map.w3u file with it.

Thank you, Mike, for so adroitly explaining that alternative that I always feel compelled to explain, even though it's not worth the effort by comparison.

Well I donno if this is the same thing, but I exported my unit settings from the map, and loaded up that .w3u file with a hex editor and I could find a text entry for Kel'Thuzad along with my other custom units but I could not find any u000 number that followed for any one.

Honestly, it takes almost no time to copy the pertinent art data from a Necromancer unit to a custom hero.

Sardius.... Create a new hero! and give him kelthuzads name and model. How hard is that.

Not very, but its just going to be annoying to have to do that for every unit who I wanna give a hero flag to to use hero spells and gain levels as a hero... Its not as quick as just changing the art model, there are quite a few other graphical factors and combat factors that need to be altered as well... Just gets tedious when you need to do it for dozens of units...

I remeber before Frozen Throne I had some modified add-on for the official editor, that would allow me to give a hero flag to any unit, but I don't believe this add on worked after Frozen Throne.