Of course you can! What we recommend is that you know how long you will need the car for and are aware of how available the owner's car is prior to booking the car. We do understand however, that in certain situations you will need to extend the rental period.
First, you must verify that the car owner is available and agrees to an extension. After this, you will have to issue a new booking that will commence at the end date and time of your current (read: original) rental and decide on your new end date/time. The car owner must approve and we will issue new documents in accordance to this and you can continue your rental hassle-free.