On Windows - if you put a shortcut to calibre.exe in the SendTo folder, you can do this:
If it's not already running Calibre will start, and the selected files will be added. I have
--with-library "D:\Calibre\Libraries\Intake" in the shortcut, if calibre is already running I make sure it's using the Intake library before I use Send To->calibre.exe.
MS should put a Send To drop down alongside Move to and Copy to for the ribbon clickers.
To open your SendTo folder, press
Winkey+R, type
shell:sendto, press OK.
BR