Hello,
do you want to extend the plugin templates with another plugin or extend the plugin templates with your theme?
If you want to extend it via your theme, you need to change the theme inhertiance as described in our documentation: Getting started with Smarty
If you want to extend it via plugin , you can find an example plugin in my github repository: GitHub - mnaczenski/MNExtendB2B
In this case it is necessary to have a look at the subscriber of the plugin you want to extend, because you need to register your plugin template before the plugin registers its template directories.
Moritz