I think you may be confusing what the purpose of having things as projects is. A project is each seperate game, for example I make a sudoku app, or an angry birds-like game, each of them is their own project. You don't import a whole project into another one normally.
I think what you are instead trying to do is instance a scene (.tscn file) under your background. You need to store all of the scenes your game will use in the same project folder.
File structure could be like this: