To reserve open segments just call the airline you want to fly on and make a reservation. Tell them you have an open "x" class ticket and give them the ticket number for their records. They will create a PNR in their system for you for the flight. If you make multiple bookings with the same airline at the same time, you will only have one PNR, separate bookings (if you don't mention the one made previously) or bookings on different airlines will result in a PNR for each booking.