Purchase date and start date of JR Pass

I plan to purchase 2 different PASSES. One is a 3 day Kanto JR Pass for travel from Narita to Karuizawa for the first 3 days. Then a 7 day JR PASS for the next seven day. Do I need to buy exchange order or do I go directly to the JR ticket office in Narita airport to buy Kanto JR Pass?
From Karuizawa I plan to travel to Hokkaido and Kyushu. I plan to buy the exchange order before I leave USA. When I get to Narita airport I will exchange for the JR PASS. At this time can I select the date I want to activate the PASS?

Ken