https://github.com/MinecraftForge/MinecraftForge/blob/1.12.x/src/main/java/net/minecraftforge/common/BiomeDictionary.java

This would allow them to be a bit more accessible in modpacks with biome-adding mods.

Ill definitely look into how to implement this.

Upon further inspection, it's already implemented! :P
However, learning of this even existing is good, because I wanted to add more spawn-able places anyways.
Expect spawning in hills biomes to be added soon.