The answer to this question depends on whether you have purchased a P+Rail ticket or a Parkingpay ticket.
Yes, for P+Rail tickets.
You can extend currently active P+Rail tickets at any time up to a maximum of seven days of validity.
No, for Parkingpay tickets.
You cannot extent currently active Parkingpay tickets. However, you can terminate currently active Parkingpay tickets early at any time. In this case you will only be charged for the actual parking duration.