Transcribing a video is good for your visitors. Some visitors have hearing difficulties, so this would help them enjoy the video, and others may be unable to access the video because of slow internet speeds or other technical issues. I don't think Google would penalize you for doing this.
My bigger concern would be if Google can't confirm that you own the rights to the video. Transcribing OTHER people's videos could be seen as stealing content (maybe; it does add value in a way that cutting and pasting text or photos does not), and that could be an issue.