JR-West All Area Pass online seats

We have bought the JR-WEST All Area Pass. The we land in Kansai, we are taking a train to Hiroshima. I don't want to book a seat for that day because I don't know how long it will take us to get through inmigration, if the plane will ariive on time, etc. But we can book it for the day that we go from Hiroshima to Kyoto.

Is it possible to do that online before getting to Japan and actually have the pass itself? If yes, how please? I have spent hours and I can't find anything.

Thanks.