You can’t convert Node subclass to Widget. If you do want to make something like Widget, try to subclass Widget class.
Widget is inherent from ProtectedNode which is also inheritent from Node. So any thing you can do with Node can also be applied to the Widget class and its subclasses.
One thing you should notice is that “when you are adding a widget subclass to a widget, it will maintains its own children widgets, while when you are adding a Node class to widget, it will be add to the scene graph”.
You can take a look at the implementation of ProtectedNode class.