You can't do that. You can create another template and load it as an add-in,
but Word will still use Normal.dot as well (what you put in the add-in will
just be added).

You can (and should) also create specific document templates for specific
purposes, and there are various ways you can start Word using one of those
templates instead of Normal.dot as the *document* (not global) template,
but, unless you modify it, the New button will still always create a new
document based on Normal.dot.

You use global templates to provide AutoText, toolbars, and macros; you use
document templates to provide styles, layout, and other formatting. Without
any add-ins, if you don't choose a specific document template, then all of
the above are supplied by Normal.dot. For more, see
http://word.mvps.org/FAQs/Customization/WhatTemplatesStore.htm