The Microsoft .NET 4.0 exam on Web development (70-519) has almost this exact question in the prep materials. The answer, according to Microsoft then, is:
- Convert the web forms website into a web application (i.e. webapp project).
- Add references to "the ASP.NET MVC 2 assemblies" in the webapp's config file.
This info is in paid-for materials my employer purchased so there is not necessarily a web page stating it this clearly to which I could link.