To confirm - what you have pulled off is the weight, all of it. What is left inside is a threaded section inside the handlebar, which I do not believe is removable (it may be, but I have no idea how you would get it out, if its even possible, without a lot of heat application).
See if you can find the same bar end mirrors to suit a kawasaki handlebar, instead of generic ones, and they will have a screw in style that doesn't have to expand to lock in, they will fasten to the thread already inside your handlebar, and that way you don't have to do any further work, just unscrew the bar end weight (which you have already done) and screw in the mirrors in place of the weight and voila! All done!
Australian ER6NL 2012 in Kwaka Green
(The naked ninja650 ABS, with throttle and ECU restrictions from factory due to license classification)