Yes, it is possible to extend your trip, however the process followed is currently the same as making a new booking.
The first step in this situation would be messaging the owner to check to see if the car would be available and open to extending.
If so, the Driver will need to put through a rental request for the gap between the initial period ending and the new proposed return time of the vehicle.
The owner would need to accept this request and the Driver would need to proceed through the steps to payment. The car would then be confirmed as extended for the extra period.