If you are asking if you can replace the 5 speed with a 6 speed the answer is yes. There is a kit for the NV5600, but it is my understanding the NV5600 are becoming hard to get and get parts for. There are some later threads about folks starting to put the G56 6 speed in with some modification of the cross member and the clutch set up. If you will search the 2nd gen forums you should be able to come up with the info.
If you are asking about swapping a 6 speed for a 5 speed, most friends do not let friends do such things. The 5 speed is a good transmission, but the difference between four and five is less desirable than the six speed configuration. Personally, I would like Santa to bring me a G56. Hope this is of some help.