I have some webforms in an Asp.Net V2.0 generated using Visual Studio 2005 using Web site technology
Want to import them in to Visual Studio 2008 set to v3.5 (Asp.Net M
You have to make sure that your project is a Web Application (and not a Website). Then, You can right-click on the file or the project and select "Convert to Web Application". If all goes well, the designer file will be created. If not, then there is a parsing error in the page.
But if you want your project to truly be a MVC application, then you should not be using code-behind files for your webpages (Views), so you need to refactor the codebehind classes into Controllers. See this article why not:
http://stevesmithblog.com/blog/codebehind-files-in-asp-net-mvc-are-evil/
Right-click your aspx file and select Convert to Web Application
.