Cant do it on the xb bikes. But it should work on the older tuber models, which I happen to have now. Feel free to pull the transmission and send it my way. They got rid of the trap door inside the primary so you have to split the case to change the trans. This also means you cant fit a different trap door to allow for the 6th gear. Maybe it could be done with custom machining to the case but I dont think its worth all of that.
I completely agree it could use a 6th gear. I think everyone here feels that way.