I have been working on CJing myself for awhile, for use with cold reading on the streets. My idea is a lot different than yours and plays more for tourists, but I like your idea.
I would imagine the best possible solution is to use a palming method of Billiards to accomplish what you're thinking about doing. I do not see how you would be able to do it otherwise.
As the ball is in motion, I do not think you will be able to make it grow, as it goes, so to speak. I would suggest, taking a look at Jeff Mcbride's manipulation series, and see what you come up with.
I could see you using misdirection, going from the Butterfly, in the windshield wiper (with some nice patter, as you stare into the ball, into the soul of the young thief...) and with your other hand palming a larger ball and then simply switching hands when you turn, as if you were going back into a butterfly, but instead going from the left to right hand windshield wiper. Boy, I bet that is clear as mud!
I will have to pull out my sphere and see, what I come up with. You may want to post this topic in the manipulation section.
Kevin