Isn't there an item on the Qt Add-in menu for "Convert to Qt project" or something like that?

At worst, you can back up your files somewhere, delete the project (in VS and manually), and re-create it. If you add classes with exactly the same names as your old project, you can copy the old files over top of the new ones and it should all work. You can also create the project, copy the files into the project directory, then use the "Add -> existing item" command from the right-click on the Solution browser to add the copied files into the project. As long as you be sure to create it as a Qt project, it should all work.

Don't add anything that is auto-generated (like moc files). For Qt GUI classes, just add the .cpp, .h, and .ui files. For QObject classes, just the .cpp and .h.