Now, when you click on a file with that extension, Visual Studio will open it using that editor. Once there, type an extension in the Extension text box in the top left-had corner, pick an existing editor from the dropdown list to the right of the text box, and click the Add button just a bit further to the right. To do that, go to Tools | Options | Text Editor | File Extension.
If you want to be able to edit it in Visual Studio, you just need to associate its file extension with one of Visual Studio's editors. If you have some file that you want to include in your project, you can add it in Solution Explorer. Despite the file extensions you see in the Add Existing Item dialog box, Visual Studio isn't limited to working with specific kinds of files.