Creating heroes isn't actually that hard using xml.

Go into Steam/SteamApps/Common/FE Legendary Heroes/data/english: Core Units

Copy A hero to My Games/LegendaryHeroes/Units

Create a sovereign using the normal method.

Replace the copied heroes internal name, display name, equipment, abilities, backstory, model, skin, soundpack, animation pack, skeleton, model type, eye texture, color_X, race type and allegiance with the data from the generated sovereign.

Modify the hero's spawn rating to an appropriate number. Adjust the unit so it's interesting/balanced/works.

I guess it's a little complicated. But... anyway... it's pretty easy as LH modding goes.