register_importer()

register_importer( string $id, string $name, string $description, callable $callback ): void|WP_Error

function register_importer( $id, $name, $description, $callback ) {
	global $wp_importers;
	if ( is_wp_error( $callback ) ) {
		return $callback;
	}
	$wp_importers[ $id ] = array( $name, $description, $callback );
}