I've encountered this, but I haven't done it myself. Creating a hero - I can assume that it simply involves creating your own files - descriptions, icons, models, and animations for the hero, and then embedding them into the game by installing the mod. There's nothing particularly complicated about it, you'll probably figure it out.
What might cause difficulties is creating your own model. You'll need to find the required model, embed it in the mod, specify the skeleton, geometry, and animations. This is probably the only thing that could take a lot of time.