Just call reservations and say that you have an existing open ticket (don't mention "round the world" or "oneworld explorer" it just scares reservations staff unnecessarily) and ask for reservations on the sectors you require in L class and give them the ticket number. Make a note of the PNR. Check in as normal.

The contract for OneWorld Explorer tickets allows for "service fees" for re-issues. A re-issue is not needed in this case since open-dated ticket coupons are explicitly allowed in the contract you have. The contract does not allow for service fees for use of open-dated segments, so refuse any attempt to charge you for "stickering".