Add External Sitemap to Yoast Sitemap Index

yoast_seo_sitemap_add_external.php

<?php
/********* DO NOT COPY THE PARTS ABOVE THIS LINE *********/

/* Add External Sitemap to Yoast Sitemap Index
 * Credit: Paul https://wordpress.org/support/users/paulmighty/
 * Last Tested: Aug 25 2017 using Yoast SEO 5.3.2 on WordPress 4.8.1
 *********
 * This code adds two external sitemaps and must be modified before using.
 * Replace http://www.example.com/external-sitemap-#.xml
   with your external sitemap URL.
 * Replace 2017-05-22T23:12:27+00:00
   with the time and date your external sitemap was last updated.
   Format: yyyy-MM-dd'T'HH:mm:ssZ
 * If you have more/less sitemaps, add/remove the additional section.
 *********
 * Please note that changes will be applied upon next sitemap update.
 * To manually refresh the sitemap, please disable and enable the sitemaps.
 */

add_filter( 'wpseo_sitemap_index', 'add_sitemap_custom_items' );

function add_sitemap_custom_items() {
   $sitemap_custom_items = '
<sitemap>
<loc>http://www.example.com/external-sitemap-1.xml</loc>
<lastmod>2017-05-22T23:12:27+00:00</lastmod>
</sitemap>';
   
/* Add Additional Sitemap
 * This section can be removed or reused as needed
 */
  $sitemap_custom_items .= '
<sitemap>
<loc>http://www.example.com/external-sitemap-2.xml</loc>
<lastmod>2017-05-22T23:12:27+00:00</lastmod>
</sitemap>';

/* DO NOT REMOVE ANYTHING BELOW THIS LINE
 * Send the information to Yoast SEO
 */
return $sitemap_custom_items;

}