If there is no conversion kit, than you could modify one B777-200 into a B777-300 by adding fuselage stretch: 5.30 meter(for 1/144 scale = 36.80 mm) from section 43 forward of the wing;
4.40 meter (for 1/144 scale=33.30mm) to section 46 behind of the wing, and extra door above the wing. Don't forget to ad tail skid. The undercarriage has to extend 4mm to allow same rotation on take-off. See update Laurent Herjean, lherjean@noos.fr
http://groups.yahoo.com/group/airlinermodelling/