No, unfortunately it is not possible to extend your booking. If you realise that you are going to stay longer than expected, and your reservation has not yet started, you can buy a new one.
Please note that if you are already in the car park, the new reservation will only work if you leave and re-enter the car park.
But otherwise you can simply stay in the car park and leave whenever you want after your reservation has ended. You will be charged the difference at the public rate and will not receive the discount.