Can a button be created to trigger Closed Caption?

Yes, this is easily done. Create your SmartShape button, then setup an Advanced Action to toggle the cpCmndCC variable. 

You'll find a solution in this blog post, not only for a CC toggle button, but also for other similar buttons:

1 action = 5 Toggle Buttons - Captivate blog

That shared conditional action is using a tracking variable, in your case this will be cpCmndCC, and a shape button (which can be used on a master slide or on the first slide, timed for the rest of the project) with one extra custom state to be able to show the status of the toggle button.