Add a custom child sitemap xml as a child to the sitemap_index.xml

Hello,

Thank you for contacting us.

You can use the below filter to add the additional sitemap in the index sitemap list:

add_filter( 'rank_math/sitemap/index', function( $xml ) {
	$xml .= '
		<sitemap>
			<loc>http://example.com/new-sitemap.xml</loc>
			<lastmod>2020-09-14T20:34:15+00:00</lastmod>
		</sitemap>';
		return $xml;
}, 11 );

Please replace http://example.com/new-sitemap.xml in the above code with the desired sitemap URL.

You can follow this guide to add the code:
https://rankmath.com/kb/wordpress-hooks-actions-filters/

Let us know how that goes.

Thank you.