Adds a new sitemap entry for Axelerant Presentation Slides into the sitemap index created by Yoast SEO plugin. `wpseo_sitemap_index` , `wpseo_enable_xml_sitemap_transient_caching`

axl-slides-sitemap.php

<?php
/**
 * Plugin Name: AXL Slides Sitemap
 * Plugin URI: https://gist.github.com/subharanjanm/885a0e7a63ad8d4f142c545221ea7e0c
 * Description: Adds a new sitemap entry for Axelerant Presentation Slides into the sitemap index created by Yoast SEO plugin.
 * Version: 1.0
 * Author: Subharanjan
 * Author URI: https://gist.github.com/subharanjanm/885a0e7a63ad8d4f142c545221ea7e0c
 * License: GPLv2 or later
 */

defined( 'ABSPATH' ) or die( "No script kiddies please!" );

function axl_add_sitemap_slides() {

	if ( class_exists( 'WPSEO_Sitemaps' ) ) {
		$plugin_file        = pathinfo( $_SERVER['PHP_SELF'] );
		$modification_time  = date( "c", filemtime( $plugin_file["basename"] ) );
		$slides_sitemap_xml = plugins_url( 'slides-sitemap.xml', __FILE__ );

		$slides_sitemap = '<sitemap>' . "\n";
		$slides_sitemap .= '<loc>' . $slides_sitemap_xml . '</loc>' . "\n";
		$slides_sitemap .= '<lastmod>' . $modification_time . '</lastmod>' . "\n";
		$slides_sitemap .= '</sitemap>' . "\n";

		return $slides_sitemap;
	}

	return '';
}

add_filter( 'wpseo_sitemap_index', 'axl_add_sitemap_slides' );
add_filter( 'wpseo_enable_xml_sitemap_transient_caching', '__return_false' );

Create an xml file with the custom urls and store it somewhere in your server. ( say slides-sitemap.xml ). Use it's path in the above code.

<?xml version="1.0" encoding="UTF-8"?><urlset xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance"
        xmlns:image="http://www.google.com/schemas/sitemap-image/1.1"
        xsi:schemaLocation="http://www.sitemaps.org/schemas/sitemap/0.9 http://www.sitemaps.org/schemas/sitemap/0.9/sitemap.xsd"
        xmlns="http://www.sitemaps.org/schemas/sitemap/0.9">
    <url>
        <loc>https://example.com/wp-content/reference/slides/</loc>
        <lastmod>2016-05-23T04:29:50-04:00</lastmod>
        <changefreq>weekly</changefreq>
        <priority>0.8</priority>
    </url>
    <url>
        <loc>https://example.com/wp-content/reference/slides/TYPO3-Best-Practices</loc>
        <lastmod>2016-05-19T09:04:58-04:00</lastmod>
        <changefreq>weekly</changefreq>
        <priority>0.6</priority>
    </url>
    <url>
        <loc>https://example.com/wp-content/reference/slides/Command-Line-for-Newbies</loc>
        <lastmod>2016-05-19T09:04:58-04:00</lastmod>
        <changefreq>weekly</changefreq>
        <priority>0.6</priority>
    </url>
    <url>
        <loc>https://example.com/wp-content/reference/slides/why-coding-php-fundamentals-short</loc>
        <lastmod>2016-05-19T09:04:58-04:00</lastmod>
        <changefreq>weekly</changefreq>
        <priority>0.6</priority>
    </url>
    <url>
        <loc>https://example.com/wp-content/reference/slides/why-coding-php-fundamentals</loc>
        <lastmod>2016-05-19T09:04:58-04:00</lastmod>
        <changefreq>weekly</changefreq>
        <priority>0.6</priority>
    </url>
</urlset>