Reserving seat before pass begins?

Good evening,

I will be doing an 8-day trip to Dewa Sanzen and Tohoku in July, so I have bought a 7 day pass. Of course, this will not get me both from and to Narita airport at both ends of the trip. I like to spend my first night in Asakusa, which is most easily accessed from NRT via the Keisei line instead of the JR lines.

If I land in NRT on Sunday and turn in my rail pass exchange order, but ask them not to star the pass until the next day, will they be able to get me reserved seat tickets for a Monday ride on the Niigata Shinkansen? This is better since I won't be using the JRPass on the Sunday when I just take the Keisei line in to Tokyo, and if my pass doesn't start until Monday, then it will be active on the Sunday when I want to go back to the airport.