Sure - I buy my books at Amazon, and have them sent to Kindle for PC. I then run K4PC (this downloads the books), run Windows Explorer, go to the "My Kindle Content" folder, and drag the new books from there into Calibre (this removes the DRM, if there is any). In Calibre, I have a quick look at the book, edit its metadata if necessary, check that the margins and line spacing, etc, are OK. Finally I plug in my Kindle and send it from Calibre to the Kindle.
This took longer to write than it actually takes to do
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