Calibre is a great for managing and converting ebooks, but it's not a full-blown editor.
If the source files are simple word documents, the easiest and fastest way to quickly create an ePub is Sigil
. Simply select all text in Word, copy it to the clipboard and then open Sigil and paste it into the default Section0001.xhtml in Book View mode. Add some metadata (title, author etc.), save it as an ePub and you're done.
(If you see unwanted page-breaks in Sigil, check your Word doc for hard line breaks and replace them with spaces.)