W2X, short for
writer2xhtml, is an extension of OpenOffice.org or LibreOffice. Free and multiplatform.
More info -in French I'm sorry- with my signature.
Joint is a photo (click twice) of the option to be selected in the export panel of
writer2xhtml (other options are available..
)
Your source file has to be in odt format. You directly export to EPUB without any code tweaking. You can check you EPUB though with Sigil.