Book seats online JR Tokyo Wide Pass

Is it possible to book seats online before actually getting to Japan? We have bought the tickets but we obviously don't have them yet.

Yes you can here:
https://www.eki-net.com/en/jreast-train-reservation/Top/Index

You can then pick up the seat reservations at a ticket office. If you buy the pass through JR East, then you can pick up seat reservations also at ticket machines.