Please see this article for a simple example of a fiddler web test plugin: http://blogs.msdn.com/slumley/pages/another-fiddler-plugin-example.aspx

The following article walks you through the process of writing your own fiddler web test plugins and shows you how you can modify what fiddler saves to the .webtest file: http://blogs.msdn.com/slumley/pages/writing-fiddler-web-test-plugins.aspx

The following article discusses enhancements made to fiddler which make it easier to use fiddler for recording/saving VSTS web tests. http://blogs.msdn.com/slumley/pages/enhanced-web-test-support-in-fiddler.aspx

This article will walk you through the process for debugging a web test. http://blogs.msdn.com/slumley/pages/how-to-debug-a-web-test.aspx