I needed to be able to add a new extension to the XML language definition in Notepad++, and found this Super User post to be simple and to-the-point.
When you open a file in notepad++ it will attempt to load syntax highlighting for the contents based on file extension. If you use custom extension for one of the existing languages you can add the custom extension in Settings -> Styler Configurator. Select language and add the extension in “User extension” edit box. To add multiple extensions separate them using space. You then need to re-open your file to see the syntax highlighting applied. You can also change syntax highlighting style by selecting a different language from language menu.
Hope it helps someone else!