You can add existing files to your project in two ways:
Right-click the Source Files folder and choose Add Existing Item. You can point to an existing file on disk using the Select Item dialog box and add the file to the project.
Right-click the Source Files folder and choose Add Existing Items from Folders. Use the Add Folders dialog box to add folders that contain existing files.
Do not use the New menu item to add existing items. The Name and Location panel will tell you the file already exists.