Add an already existing directory to a directory in Solution Explorer

I want to add an already existing directory to a directory in Solution Explorer, but whenever I right-click on the directory and select Add => Existing Item, I can only add individual files, but not directories.

How do I add an already existing directory to a directory inside a Project inside Solution Explorer?

Click the 'Show all files' button at the top of the Solution Explorer and right click the folder desired and select 'include in project'.

Drag and drop the folder from Windows Explorer onto your Visual Studio solution window :)

Source here

or simply copy & paste into solution explorer.

VS 2012 seems to distinguish between 'Solution Folders', which are only folders containing either other solution folders, or containing project folders. The drag-and-drop works (with my settings) only for the project folders, and no for the solution folders. If I add a new solution folder, nothing happens on the machine. If I drag-and-drop a machine folder to the main Solution, it refuses to accept it. If I drag-and-drop the folder to a Solution Folder, I get an error message saying this cannot be done.