add_feed( string $feedname, callable $function )

function add_feed( $feedname, $function ) {
	global $wp_rewrite;

	if ( ! in_array( $feedname, $wp_rewrite->feeds ) ) {
		$wp_rewrite->feeds[] = $feedname;
	}

	$hook = 'do_feed_' . $feedname;

	// Remove default function hook
	remove_action( $hook, $hook );

	add_action( $hook, $function, 10, 2 );

	return $hook;
}