It is possible to fit a throttle by tapping into the cable linking the display and the controller fitted near the bottom bracket and
I have done so.
See post 8 in the attached link for circuit schematic & background info.:
Moin, ich habe bereits im dem 'anderen' pedelec und E bike Forum meine Untersuchung und Ergebnisse veröffentlicht, hoffe aber hier zusätzlich noch weitere interessierte Leute zu erreichen. Zunächst einmal ein Dank an Arthus Dent der schon vor über einem Jahr damit angefangen hat und auf dessen...
www.pedelecforum.de
Google translate will deal with the language if your German is not up to it!
You need to put 5 v (from the purple and black wires across the throttle Hall sensor and attach the Hall sensor signal wire to the yellow line. If you have a voltage display on your throttle and it has the range up to at least 60 v, you could also tap into the red wire of the cable to display the battery voltage.
Unfortunately all you achieve with this, is having the "walk" function (ie 8 or is it 6? kph) available via the throttle.
It should be possible to increase the speed available by resetting the firmware but this is beyond my ability & competence!
I have posted (under the same nom de plume as I use here) in the link given above but no usable feedback has been provided. If anyone reading this & can help with this, please post here or send me a pm.