How to create a component with existing entities in a component by ruby?

Here is a non-method example. This example uses a mouse context menu command block:

UI.add_context_menu_handler do |popup|
  sel = Sketchup::active_model.selection
  if !sel.empty? && sel.single_object?
    if sel.first.is_a?(Sketchup::ComponentInstance)
      popup.add_item("Copy Selected Component to Origin") {

        Sketchup::active_model.active_entities.add_instance(
          sel.first.definition,
          IDENTITY 
        )

      }
    end
  end
end

Point to a component instance, right-click the mouse, and you should see the menu command at the bottom of the mouse context menu: