Your only hope, from what I know, is that if you can run an Envoy Reader you could print them using a "print to PDF" printer driver. The resulting PDF could either be viewed as is or used as the next step in a conversion process (convert to editable format like ePub in Calibre, edit ePub in Sigil to fix conversion flaws, convert to final desired format in Calibre).
One such PDF printer driver that I like is PrimoPDF:
http://www.primopdf.com/index.aspx