It's possible if you still have your kindle, it works, and is registered to your amazon account, but it involves a bit of work.
The steps are:
download to your kindle over wifi or copy over USB
open and close the book on the kindle and copy the book.apnx file in the book.sdr directory on the kindle to your computer
pass the book through calibre, then use kindlunpack to convert the book and associated book.apnx to EPUB
email the EPUB to your kindle library
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