SDK Question

You can add objects, yes. You can't remove objects or remove earth, but you can add objects, including earth-like objects. You have to use the objects in the pre-defined level object library, since those are the only objects your game is coded to have access to. You place the objects in godot visually, export your changes to a json file which is just a list of the objects and their location, size, and orientation, and then import that into the portal web publishing tool to tell your game server what to tell the player's computers to display inside their games.