How do I extend the xmlsitemap module to include the google news / images and videos formats
For news there's the Google News sitemap module


  The Google News module generates a Google news compatible sitemap feed from your Drupal site's content.


I don't think there are existing modules for images/video (I've looked for the same thing before myself and ended up hacking something together).

There's a feature request in the XMLSitemap module's issue queue for exactly this, but it's currently postponed. There are a couple of links on that thread pointing to how others have achieved it, those might be worth a look.