you can install a 6 speed tranny
From reading the cars description it looks like the us versions (IS) have 5 speed automatics while all of the japanese models (Altezza) had 6 speeds.
Edit: I took out a few and tested them, The IS 200 (J) '98 and the Altezza RS200 have 6 speeds while the IS 200 '98 and the Altezza AS200 have 4 speeds as stock.