Custom Wither Spawning

I'm making a mod that, along with other things, changes how the wither spawns. Basically, instead of using four soul sand and three wither skulls, it replaces the center soul sand with a different block (like how it uses a command block in the middle for Minecraft Story Mode's wither storm). Is there any way to change the blocks/ order of blocks needed to spawn the wither in base MCreator? Or would I need a plugin? Or would I need to dig into the game's code? Thanks!