You cannot add an existing folder, but you can add several existing items at the same time. When you select "Add Existing Elements", you can select multiple files in the dialog box. By default, this will copy the item to the project directory if it is outside of it.
If you do not want to add the actual files to your project as a copy, you can add them as a link.
Add a new folder to your project. Then select "Add an existing item ...", go to the folder with several levels up, select all the files in the folder that you want to add, and click the small drop-down button on the "Add" button. Then select "Add as link."
alt text http://img19.imageshack.us/img19/4343/9132009103548pm.png
Files will be associated with your project, but will not be physically copied. Make sure your namespaces are correct.
source share