You first have to convert that web-site into a web application. When you right-click on the web-site node in Visual Studio's Solution Explorer, you should have an option "Convert to Web Application". This is the first step.
The next step is to change the project GUIDs from a basic ASP.NET Web application into MVC application guids, so you get right-click support for adding controllers and views.
Note:
Please dont go with conversion because there are lot of difference between classic asp and MVC So you better rewrite in MVC to avoid lot of errors.