"Select Existing Package" is related to creating models.
I assume it's not what you're talking about. If you want to add objects to an existing model, either open an existing project for the model or create a new project and make sure its for the model where you want to add things. (You can change it in model properties.
You'll use "Select Existing Package" only if you want to create more models inside the same package, which you won't do often.