As long as you setup the routing in web.config, setup the necessary directory structure, and add the correct routes in global.asax, you could theoretically add MVC items to any web project. So far as I know, those are the only requirements for it to work.
However, the combination of the two might be a bit confusing and difficult to maintain, long term. Maybe you could move all of the existing web forms site content into a subfolder to keep it out of the way and keep the root directory of the site clean to reduce the clutter and make things more clear.
