How to pin ANY Modern app, not just the built-in ones to the Taskbar:
1. Download OblyTile from http://win8tiles.com/Home/OblyTile
2. Click the ‘Open Tile Manager’ button in its top right corner.
3. In the ‘Manager’ section, click the ‘Create shortcuts for Windows Apps’ button (the button with the Windows logo).
4. Select the App for which you want to create a shortcut on the Desktop and click the button below called “Create shortcut for the selected app”
5. The shortcut will be created on the Desktop. Give it an icon of your choice by right clicking it, Properties -> Change Icon.
6. Now right click the shortcut and click “Pin to Taskbar”.
That’s it. You have Metro Apps launchable directly from the Taskbar.