Add additional templates to the templates drop down on add page menu [duplicate]

Do you want to add this template via a plugin or using the theme? If you want to use the theme you need several things:

1. The PHP file which will be responsible for the page. The file itself can be named in two ways: page-{slug}.php or page-{id}.php. Here is an explanation what each one of them does.

2. You need to add the template name. Right after the opening <?php tag you need to add:

   /* Template Name: Your Template Name */

This name will be also used in the drop down.
3. You need to code the logic for the template.

Here is a good article on page templates.