If you want to customise your very own hero but you just want the gear and stats from one of the stock heroes, do a Ctrl-F (Find) in CoreUnits.xml, highlight from <UnitTypes> to </UnitTypes> and copy, then paste into Notepad.
Design the look of the hero as a sovereign in-game, exit the game, open that file and copy just the parts which relate to appearance, paste over the appearance section in your new hero file and save as before with an XML extension.
It wouldn't hurt to come up with your own backstory as well but I suppose if you're in a hurry and just want to see if this works, you can do that later.
Obviously, as I said before, it would be a lot quicker to be able to do all these steps in-game. You never know, it might make it in there, looking at the release date I'm not sure it will scrape into a 1.0 release but you can look forward to some more updates after that.
Thing is, not everyone gets why this is needed. I'll be blunt and say that after seeing some of the heroes on the select screen I'm just itching to create more because the choices are often unsurprising. However even if I had the time I still have to go through that horribly complicated process I just wrote, unless I know exactly what appearance to use in which case I save myself one trip into the game.
Long story short, when you're finished making some new heroes for yourself, throw us some screenshots of them and maybe the mass of players will start to demand this feature as well. 
