First you need to understand that everything is a node.
The "Scene" itself is a node. The "Player", too.
So what you are looking for is to add a node to an other node as a child.
So more like spawning.
var myNode = preload("Drag and Drop type your path here")
func add_myNode _to_my_game():
var myNode _instance = myNode .instance()
#set position if needed
myNode_instance.global_transform = global_transform
Thats how you can add a node to the "furthest up" node via script for example.