The validity of Japan Rail Pass is 7,14 or 21 consecutive days, it is valid for the railways, certain buses, and JR Miyajima ferry.
Once you will get your Japan Rail Pass Exchange voucher, you will have 3 months to exchange it. To Exchange your voucher you must visit an Exchange Office which can be found only in Japan. At the exchange office, you will have to indicate the date you want your Japan Rail Pass to be activated. The date you choose will be the date you will be able to use the pass. Once you have the date on your JRP, it cannot be changed.
The JR pass is only valid for a person indicated on the pass, it is not possible to transfer it to another person. You must carry your passport or an ID card while traveling on JRP, to be able to show it to the station officer if required.
Once you have your JR voucher, It is not possible to add more days to your pass. If you want to extend it, you will have to purchase a new pass. As mentioned before, the pass is only available for 7, 14 or 21 days and it is not possible to extend it for a few days more.