late activation of JR Pass

Hi there,
I will stay in Japan for 17 days, my plan is to arrive to Narita airport and head with Narita express to Tokyo, my first usage of JR Pass is planned to be 3 days later in Hikari from Tokyo to Osaka and last on the day before last day of stay (Nagano to Narita). Can I activate the JR Pass on the first day I will use it? Is there such a possibility in Tokyo station?
Thanx.
L