add_filter( 'aioseo_sitemap_indexes', 'aioseo_add_sitemap_index' );

function aioseo_add_sitemap_index( $indexes ) {

	$indexes[] = [

		'loc'     => 'https://somedomain.com/custom-sitemap.xml',

		'lastmod' => aioseo()->helpers->dateTimeToIso8601( '2021-09-08 12:02' ),

		'count'   => 1000

	];

	return $indexes;

}