The A330-300 is built with the same centre tank structure as the longer-range A330-200 but is unused.
Airbus will modify the aircraft on the production line by sealing the centre section, making the volume available for fuel transport.
It will also install associated pumps as well as the inerting system required by US and European regulators.
Airbus expects the increased fuel capacity, combined with planned aerodynamic tweaks, to bring the maximum take-off weight to 242t and increase the range of the A330-300 by 500nm (930km) to 6,100nm.
It claims that while the 787-9's cash operating costs per seat would be 6% lower its direct operating cost per seat will be 7% higher than the 242t A330-300's. Boeing gives the range of the 787-9 at 8,000-8,500nm.
Airbus is also positioning the type as a Boeing 777-200ER replacement, claiming the aircraft will increase its proportion of network coverage from 85% to 94% of the -200ER's. Over 400 -200ERs, which have a range of 7,725nm, have been delivered since the first in 1991.