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I think that Amazon supplies a copy of the book from the Amazon archive. The Amazon archive is just the books you've bought from Amazon--the same list you can see on the "Manage your Kindle" screen. I would bet that no electronic copy of the book is stored in your "archive" at all-just the information Amazon needs to access the file and add the right PID when it send the file to a device.
If all of these assumptions are true, there is no way to add external files to the archive.
You can, however, "publish" your ebook at Amazon using their self publishing tools, price it at zero and buy it. I'll bet that book will sync!!!
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