How to add existing folder with files into Visual Studio?
From the top of the solution explorer window. You will find the button to "Show All Files" for the specific project (you might need to hit "Refresh" to see them). The folders/files that are not part of your project appear slightly "lighter" in the project tree. Right click the folders/files you want to add and click "Include In Project". It will recursively add folders/files to the project.