There is no out-of-the-box support for migrating from setup projects to Windows Installer XML toolset projects in Visual Studio 2010. This could be part of a future release though.
As PhilWilson mentioned, the best place to start is dark. Dark.exe is part of the public Windows Installer XML toolset; Dark converts MSI and MSM files into a WiX source file. However, the output still needs to be “tweaked” to get to the desired authoring. After cleaning up your authoring, all what you need is to include it into a newly created WiX project in VS and you are good to go. Modulo custom actions, you should easily be able to get a working Windows Installer XML toolset project from your current setup project.
For more information about dark, go to: http://wix.sourceforge.net/manual-wix2/dark.htm
Mohamed Hegazy