Append a Page to the Additional Pages Sitemap

add_filter( 'aioseo_sitemap_additional_pages', 'aioseo_sitemap_add_additional_pages' );

function aioseo_sitemap_add_additional_pages( $pages ) {

	$pages[] = [

		'loc'        => 'https://example.com/additional-page',

		'lastmod'    => '2020-12-11 11:11',

		'changefreq' => 'always',

		'priority'   => (float) 1.0

	];

	return $pages;

}