The only way I'm aware of is using good ole copy/paste. There is no capability I'm aware of that allows transfer of anything from Captivate back to PowerPoint. It's always a one way flow. From PPT -> Captivate.
Agree with Rick.
You can make the slide notes as Audio CC, export it as a Word doc, and copy-paste it from there. Perhaps, it might be easier than copying notes from individual slides in Captivate. But slide content will still have to be copy-pasted from Captivate only.