WordPress Yoast SEO: Create a custom sitemap with data not sourced from a custom post type.

wpseo-yoast-sitemap-custom.php

<?php

/**
 * USAGE:
 * - Search and replace the `CUSTOM_SITEMAP` string with a lowercase identifier name: e.g., "vehicles", "customer_profiles"
 * - The `your_alternate_data_source()` function does not exist; it's simply a placeholder for your own function that returns some sort of data array. 
 * - Uses heredocs for inline XML: https://www.php.net/manual/en/language.types.string.php#language.types.string.syntax.heredoc
 */

/**
 * Uncomment the next line of code to disable sitemap caching.
 * - For development environments and debugging only. 
 * - All other scenarios, follow the "Manual Sitemap Update" instructions in the Yoast documentation:
 *   - https://yoast.com/help/sitemap-does-not-update/#manual
 */
#add_filter("wpseo_enable_xml_sitemap_transient_caching", "__return_false");

/**
 * Add CUSTOM_SITEMAP-sitemap.xml to Yoast sitemap index
 */
function CUSTOM_SITEMAP_sitemap_index($sitemap_index) {
    global $wpseo_sitemaps;
    $sitemap_url = home_url("CUSTOM_SITEMAP-sitemap.xml");
    $sitemap_date = date(DATE_W3C);  # Current date and time in sitemap format.
    $custom_sitemap = <<<SITEMAP_INDEX_ENTRY
<sitemap>
    <loc>%s</loc>
    <lastmod>%s</lastmod>
</sitemap>
SITEMAP_INDEX_ENTRY;
    $sitemap_index .= sprintf($custom_sitemap, $sitemap_url, $sitemap_date);
    return $sitemap_index;
}
add_filter("wpseo_sitemap_index", "CUSTOM_SITEMAP_sitemap_index");

/**
 * Register CUSTOM_SITEMAP sitemap with Yoast
 */
function CUSTOM_SITEMAP_sitemap_register() {
    global $wpseo_sitemaps;
    if (isset($wpseo_sitemaps) && !empty($wpseo_sitemaps)) {
        $wpseo_sitemaps->register_sitemap("CUSTOM_SITEMAP", "CUSTOM_SITEMAP_sitemap_generate");
    }
}
add_action("init", "CUSTOM_SITEMAP_sitemap_register");

/**
 * Generate CUSTOM_SITEMAP sitemap XML body
 */
function CUSTOM_SITEMAP_sitemap_generate() {
    global $wpseo_sitemaps;
    $data = your_alternate_data_source();  # Replace this with your own data source function
    $urls = array();
    foreach ($data as $item) {
        $urls[]= $wpseo_sitemaps->renderer->sitemap_url(array(
            "mod" => $item->date,  # <lastmod></lastmod>
            "loc" => $item->url,  # <loc></loc>
            "images" => array(  
                array(  # <image:image></image:image>
                    "src" => $item->image_url,  # <image:loc></image:loc>
                    "title" => $item->image_title,  # <image:title></image:title>
                    "alt" => $item->image_title,  # <image:caption></image:caption>
                ),
            ),
        ));
    }
    $sitemap_body = <<<SITEMAP_BODY
<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 http://www.google.com/schemas/sitemap-image/1.1 http://www.google.com/schemas/sitemap-image/1.1/sitemap-image.xsd"
    xmlns="http://www.sitemaps.org/schemas/sitemap/0.9">
%s
</urlset>
SITEMAP_BODY;
    $sitemap = sprintf($sitemap_body, implode("\n", $urls));
    $wpseo_sitemaps->set_sitemap($sitemap);
}

Hi how to add double image ? E.g product variants have photo ids like 11111,22222,33333 in the 'product_image_galery' meta. How can I add?

If you're talking about Product Gallery Images, then you can achieve this by:

$attach_ids= "11111,22222,33333";
update_post_meta( $product_id, '_product_image_gallery', $attach_ids);

NOTE: The $product_id would be your parent product id.