There is no official means to mod Slime Rancher at this time and we do not officially support it. You are free to do it on your own, but by modding your game you may be forfeiting support from Monomi Park if you experience technical issues.
Additionally, we do not approve of mods which make unreleased content available. We're working very hard to deliver exciting content updates to our players and distributing mods that allow access to incomplete versions of this content is harmful for us and for players waiting patiently for it.
We don't want to give you the impression that Monomi Park dislikes mods or modders, but, simply put, mods greatly complicate the way we are able to support the game and ranchers playing it. Though we may support modding one day in an official capacity, we are currently focused on delivering the game we've planned for our players. We hope you understand.