Many rental car operators will allow you to purchase additional days and kms as long as you stay within the dates on the listing. However they will either say this on the listing details or in their terms and conditions and will give you a daily rate. Additional kms will always be available to purchase and can be up to 40c per km. If additional days are not stated in the comments section of the relocation listing, then additional days are not allowed. If you go ahead and request a vehicle and get accepted, you will be required to take it regardless of if you can purchase additional days or not and must stay within the dates and allowances specified.