Use the sgg_additional_index_sitemaps filter to include additional Sitemap URLs in the Sitemap Index. This filter allows you to append custom Sitemap URLs to the existing Sitemap Index.
Example usage:
add_filter('sgg_additional_index_sitemaps', 'add_my_custom_sitemaps');
function add_my_custom_sitemaps( $sitemaps ) {
$sitemaps[] = 'http://example.com/custom-sitemap.xml';
return $sitemaps;
}
Replace https://example.com/custom-sitemap.xml with your custom Sitemap URL.