Add youtube captions to existing posts.

Hello, I already have many posts (10,000+) on my website with youtube videos on them but they don't have captions added in text as content for the post. As such, I created a custom field for the (10,000+) existing posts where I will add youtube captions. I'm just wondering if there is a way to use the captions generated by the youtubomatic plugin to update existing posts (i.e. append the youtube captions generated by youtubomatic to the custom field area of the existing posts). The urls of the existing post will be the same as the urls of the aiomatic posts, since both are based on the youtube video title. So I thought the two could be matched together using regex expressions. It's just that you also have to look within the existing post, find the new custom field where captions will go, and add the generated youtubomatic content there. I don't know if this is possible or if the regex settings only allows you to find and replace fresh content generated by youtubomatic, rather than append it to old existing posts. Thank you in advance