You can't get existing UI elements directly, but you can still add UI elements to existing elements
You'll have to use the Internals class for this. By default you'd use the Player.addUIElement() method to add the element to a player screen. This method also has an overload which accepts a UITarget, but this only supports a few, pre-defined targets.
However, if you want to add a UI element to an arbitrary existing element, you should use the Internals.addUIElementToPlayer() method instead to add the element to the player screen. Just provide the path of the target element as parameter
Example:
But the location label is a tricky UI element... not sure if you will get satisfying results when attaching childs to it...