Can I reserve seats before I activate my JR Pass?

Hi,

I would like to know if I can reserve Shinkansen seats before I activate my JR pass?

For example, I want to activate the pass on Friday, but can I reserve seats beforehand on Tuesday? Or do I need to activate the pass before I can make seat reservations?

This is because I will require the seats with large luggage allowance, so I would like to reserve them in advance in case they are unavailable. I will not need to use the JR Pass until Friday as I will only be travelling within Fukuoka from Tuesday to Friday.

Thank you!