I would like to know this as well, me and a friend had some fun ideas for custom biomes with unique trees and blocks but no idea how to make them/implement them into the game.
I want to know it too but maybe it Will help by looking How the other biomes are Made and otherwise read the Starbound detailed modding e-book.
Try not to necro a 4 year old post https://community.playstarbound.com/threads/adding-biomes-101.101968/