use of closed captioning if text is already on the screen? Transcribe feature?

I'm not sure if this helps with your project -- I'm assuming you have an existing voiceover audio track that you're looking for a way to expedite the CC'ing?  In that situation, I put the text for the voiceover into the Notes.  Then choose Insert/Audio/Text-to-Speech, and click the "Copy from Slide Notes" button (or you can just paste the text in from wherever is most convenient).  Be sure to check the box "Generate Closed Captions".  You'll get a new audio track with a computer voice and closed captioning.  Select it, than right-click and choose accessibility. Then chose the Export Closed Captions button and save the .vtt file into the project folder. 

Then, get rid of the computer voiceover track. and select your existing narration instead, right-click, and choose Accessibility.   Import the Closed Captions file you just made.  Then click on Edit and it's just a matter of repositioning the already-created captions.

A final note is that if you chose the computer voice that talks as fast or a little faster then your existing narration, you'll have the least work to do in repositioning the narration.