There are a few options available, but installing the Cruise Drive six speed from the '07 and later models isn't one of them. You can buy a Screamin' Eagle six speed overdrive gear set that can be installed in your existing transmission case, or a complete trans including the case. You can buy a Baker, either the overdrive six speed like the SE unit or the Direct Drive six speed with a 1:1 top gear ratio and a different primary ratio to achieve the overdrive effect. That's the one H-D copied when they came out with the Cruise Drive.
There are other alternatives that could also work in a relatively light bike like the Night Train that wouldn't add any more gears. Changing just the primary ratio (engine sprocket/clutch basket sprocket plus chain), or changing the final drive ratio (trans output sprocket or rear sprocket plus new belt) might make enough difference to satisfy your needs.

Jerry