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Crea un nuevo objeeto Db y pásale los datos de conexión:

$conex = new Db($server, $user, $password, $database, $connect = true);

PRESTASHOP 1.6

Lo primero que haría sería consultar la clase de conexión a la base de datos (classes/db/Db.php), método getInstance:

    public static function getInstance($master = true)
    {
        static $id = 0;

        // This MUST not be declared with the class members because some defines (like _DB_SERVER_) may not exist yet (the constructor can be called directly with params)
        if (!self::$_servers) {
            self::$_servers = array(
                array('server' => _DB_SERVER_, 'user' => _DB_USER_, 'password' => _DB_PASSWD_, 'database' => _DB_NAME_), /* MySQL Master server */
            );
        }

        if (!$master) {
            Db::loadSlaveServers();
        }

        $total_servers = count(self::$_servers);
        if ($master || $total_servers == 1) {
            $id_server = 0;
        } else {
            $id++;
            $id_server = ($total_servers > 2 && ($id % $total_servers) != 0) ? $id % $total_servers : 1;
        }

        if (!isset(self::$instance[$id_server])) {
            $class = Db::getClass();
            self::$instance[$id_server] = new $class(
                self::$_servers[$id_server]['server'],
                self::$_servers[$id_server]['user'],
                self::$_servers[$id_server]['password'],
                self::$_servers[$id_server]['database']
            );
        }

        return self::$instance[$id_server];
    }

La conexión la hace a través de las constantes que se configuran en prestashop. Prueba a hacer un override de esta clase y meterle un nuevo método, con lo que te podría quedar algo así:

    public static function getInstanceAux($master = true,$user,$pass,$database,$server)
    {
        static $id = 0;

        // This MUST not be declared with the class members because some defines (like _DB_SERVER_) may not exist yet (the constructor can be called directly with params)
        if (!self::$_servers) {
            self::$_servers = array(
                array('server' => $server, 'user' => $user, 'password' => $pass, 'database' => $database), /* MySQL Master server */
            );
        }

        if (!$master) {
            Db::loadSlaveServers();
        }

        $total_servers = count(self::$_servers);
        if ($master || $total_servers == 1) {
            $id_server = 0;
        } else {
            $id++;
            $id_server = ($total_servers > 2 && ($id % $total_servers) != 0) ? $id % $total_servers : 1;
        }

        if (!isset(self::$instance[$id_server])) {
            $class = Db::getClass();
            self::$instance[$id_server] = new $class(
                self::$_servers[$id_server]['server'],
                self::$_servers[$id_server]['user'],
                self::$_servers[$id_server]['password'],
                self::$_servers[$id_server]['database']
            );
        }

        return self::$instance[$id_server];
    }

 NO TESTEADO