While ofc plastic endcaps wont stretch so they're out - you can get a soft bicycle grip over the throttle tube w/some effort. The easiest way I've found is to dissemble the throttle completely so ya have just the tube & soaking the grip in hot, slightly soapy water.
While trying to force the grip onto the plastic kit throttle tube, it's best if you slide it onto the handlebar for support, it's fragile and you could break it otherwise. The reason to disassemble it is so when the grip cools you can throw the whole thing back in the bucket to heat up again
Stock squishy Schwinn grips;