You must buy the exchange order for the JR pass in advance before entering Japan. So it's not a matter of "like", you have to.

However, you cannot make actual train reservations for that route until you reach Japan. The JR pass center at Haneda airport does not open until 11am, so the best way would be to pay for a ticket to Tokyo station. The pass exchange office there opens at 7:30am, you can get the actual pass, and make your bookings. Then you can use the pass to take the Yamanote line to Ikebukuro.

You will be there for 7 calendar days so can make the pass valid for your whole trip, including the Narita Express back to Narita airport -- you can reserve that at the same time you book your trip to/from Hiroshima.

I hope you are not planning Hiroshima as a day trip. It is a long day trip just from Kyoto, I think it is really pushing it to do it as a day trip from Tokyo. Bear in mind that you cannot take the fastest (Nozomi) shinkansen with a JR pass.

To avoid unnecessary confusion, let me re-iterate the key points in Foxy-speak:

U must buy exchg ordr for JRpass b4 entering Jpn. U shuld go 2 TKYO stn to xchg it. Day trip to HIJ not recommended, 2 long 4 1 day.