Adding QAT buttons to a specific document or template is much easier than is doing so on a Ribbon Tab.
Modifying the QAT in Microsoft Word

Adding a custom ribbon tab to a document or template is certainly something that can be done, but not through the Word interface. It will require XML modification together with vba work.
Customize the Ribbon (It doesn't take rocket science)

For more, see RibbonX: Customizing the Office 2007 Ribbon by Robert Martin, Ken Puls, and Teresa Hennig
This says 2007 but virtually all of it applies to Word 2016.