Making Reservations Before Activating Pass

Hi,

I was wondering if I would be able to make reservations for the Shinkansen before activating my pass. I plan to take the shinkansen on April 16 and bought a 7-day pass. So I was planning to start it on April 16 and use it for my Shinkansen back on April 22. This would make it the full 7-days. Could I make a reservation before my JR pass is fully activated or will I be charged the full price of the shinkansen?

Thank you!