The only thing I can think of is that since epub files are just zip files (with a mimetype file at the beginning, and so on) and that zip files have comments, you can add something to the zip file comment.
Since I don't know what sort of computer you have, it might be best to google for something like "adding comments to zip files".
But I don't really understand what you're trying to do. Don't you want the series information inside the book's OPF file along with the other metadata?
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