[dcb]

Basically the shifts are controlled by a drum that rotates. Each gear HAS to have a point where it is in "neutral" between gears, otherwise something would break, so you figure out which shaft has the neutral detent on it, and figure out how many degrees that shaft turns per shift and put your additional detent slot(s) at those intervals from the existing slot.

For starters, you should be able to put the bike on centerstand and put it in top-gear, then by hand do a "half downshift" and get the rear wheel to freewheel (by hand). Might take a few tries.

On my bike I can unscrew the detent ball and look at the shaft, so I could possibly mark it there (or at least get in the ballpark), but you do have to be pretty precise with locating the detent groove. With a removable ball, I could manually find the "best" high neutral by hand, then put some prussian blue on the detent ball and screw the ball holder in then out and I should have a blue mark precisely where my second detent should go.

It "may" be possible to do it externally, by having a "stop" for the gear shift lever that you engage with some hand control or switch or something. For example, imagine a substantial bolt (so you can adjust it easily) that you can swing (accurately and repeatably) into the way of the downshift lever with a small hand lever or switch. If you want to go to neutral then move the bolt in the way of the shifter and hold the "half shift" down on the stop with your foot. No promises, just thinking out loud.