With Bug 191632 being implemented, it's now much easier to add your own templates: just stick them in ~/Templates! …However this isn't discoverable at all. I would recommend adding a small UI to the "Create New" menu itself to expose this: a menu item that says "Add Your Own…". Clicking on it would open a file dialog prompting the user to select a file. Ideally something in the dialog would also inform the user that their file will be copied to ~/Templates so they can manage it again in the future.