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Kay - I open DOCXs in Word from within calibre most days of the week, and I've been doing so ever since Kovid released the DOCX Input module in calibre version 2.something. I often have more than one DOCX open, books as big as, and including, Tolstoi's War and Peace.
Calibre supports .DOCX because it's predecessor .DOC, which was never supported natively in calibre, was a proprietary mess. DOCX is an implementation of the ISO Office Open XML standard, which is also supported by other applications such as Libre Office Writer, Abbyy Fine Reader etc.
I am surprised to hear Smashwords only supports .DOC, what have they been doing in the years since 2007! You might want to check that. Good Lord, I just looked them up on WikiP - they didn't even exist in 2007
BR