add_action('init', 'enable_custom_sitemap');
function enable_custom_sitemap(){
    global $wpseo_sitemaps;
    $wpseo_sitemaps->register_sitemap('xml', 'sitemaps/text.xml');
}
	
function wpseo_add_page_to_xml_sitemap( $content ) {
    global $wpseo_sitemaps;

    $content .= $wpseo_sitemaps->sitemap_url( "http://example.com/custom.xml" );
    return $content;

}
add_filter( 'wpseo_sitemap_page_content', 'wpseo_add_page_to_xml_sitemap' );