Hello,
Please check the documentation available on https://docs.joomla.org/Using_own_library_in_your_extensions , that details how to add and use external libraries with Joomla, including PHP namespace packages.
If you don't need to load the library in all pages, you could also include the main class and use it with the namespaces (using the namespaces package, not the classic one):
require_once 'path_to/Classes/Phpdocx/Create/CreateDocx.php';
$docx = new Phpdocx\Create\CreateDocx();
Regards.