[Other] Mega Evolutions in HG/SS

I am currently hacking HG/SS ROM, changing the evolutions of Pokemons. My question is: How do I "add" a Mega Evolution (For example: Mega Venusaur) to the Pokémon list? And give it a sprite. Like making a new Pokémon. I was planning on making the mega evolutions happen by leveling up or by using an item. I'm not quite sure how to do the second one, though. Would someone teach me? Or tell me what program do I need. Thanks!

I currently have one that lets me edit evolutions, abilities, base stats...