If you have created other invoice templates for your site, you will need to create a module to implement this hook so that you can choose those templates in the UI.
If your site doesn't have a custom module for this kind of hook, and you only have one custom invoice template, you can override uc_order-customer.tpl.php instead. Be sure to check your Conditional Actions that email invoices and change their settings to use the "customer" invoice template."
So, is not about adding new file, either you create module for this or override the existing template.