Okay, go to the object editor, and to the unit editor, then click the "Create new Unit button". Find the hero you want to base your hero on. Such as, the blademaster or something. Then click OK.
Go to "Edit Unit Model" in the new unit's editor thingy, and select the model of the unit you want to make a hero. THen Voila!
Hope that helps. If you need more help, just ask!
-Anomaly