Use same page-mypage.php template for several pages

To re-use the same template for different kinds of pages you can use the template_include filter.

Example given from the docs:

add_filter( 'template_include', 'portfolio_page_template', 99 );

function portfolio_page_template( $template ) {

  if ( is_page( 'portfolio' ) ) {
    $new_template = locate_template( array( 'portfolio-page-template.php' ) );
    if ( !empty( $new_template ) ) {
      return $new_template;
    }
  }

  return $template;
}