Originally Posted by jobstfeld

You could call the Japanese access number.

In my experience, you can't add the return flight to an existing booking. There is a more complicated solution, though.

1) Book outbound leg as one-way
2) Book return leg once available - get on the website and BUY the seats. You will be cancelling them soon.

Once you have the bookings, you CANNOT merge the bookings. Instead, you need to cancel the bookings and then rebook. So, cancel the outbound leg - this will also return the AMEX 241 voucher to you. You need to log out of the website and then log back in in order to see the voucher. You then cancel the inbound leg. As soon as the confirmation comes through, the seats are available (out and inbound) and the system will then be able to see the seats. If the inbound has not been cancelled, the 'in and out' legs are not available.

NOTE on seat booking. If you intend to do the steps above DO NOT RESERVE SEATS ON THE INITIAL HOLDING BOOKINGS!!!! Only reserve seats on the final booking. You will pay and there is no refund (afaik) if you cancel the booking.

HTH