It should be possible
if you have the twist throttle and don't mind losing the light switch and the horn button.
The momentary push switches in the horn/cruise unit are interchangeable with the on/off switch in the twist throttle but you would need to use the cable and connector plug supplied with the horn/cruise button unit.
Please Note: the horn/cruise switches are
not interchangeable with the on/off switch in the thumb throttle.
Start by removing the wiring and switches from the horn/cruise button unit:
The horn switch switch has to be removed and the two unused horn wires must be safely insulated:
Carefully remove the cover from the twist throttle unit and then remove the lighting switch
(leaving the red button behind):
It comes out easier if it is pressed in to the on position before pulling it away from the button.
The on/off lighting switch then has to be unsoldered, and the wires must then be carefully insulated. If you already have a light wired to the lighting output you will need to solder the two wires together before insulating them so that the light is on all the time.
Fit the cruise switch into the throttle unit housing and carefully tuck the insulated unused wires neatly out of the way as space is very limited in the housing before passing the cable through the additional hole in the throttle casing
(circled in yellow in the above photo) and refitting the cover.
I suspect that the additional hole in the cover is to allow any water to drain out of the throttle housing, so it might be advisable to drill another similar sized hole
(at the lowest point) for this purpose.
The following picture shows where the additional cruise\horn cable exits through the hole in the cover, but I did not actually remove the horn and lighting switches from my wires as this was just a test to see if it would work:
The plastic cover is an absolute pain to refit, as the twistgrip section
(which I could not remove without the risk of breaking it) gets in the way, but I did eventually manage to fit it back after a lot of fiddling around.
So, the answer to your initial question is yes, it can be done, but it's not quite as straightforward as you might imagine.
I'm not sure how easy it would be to hold a steady part throttle position whilst simultaneously pressing the cruise button with the same hand, but engaging the cruise function at full throttle should not be too hard
Alan 
