The latest beta version
of Atlantis Word Processor 18.104.22.168 has a new "batch conversion" command. You can use it to convert many documents to the EPUB format at one go.
EPUB is not the only output format offered by this "batch conversion" command. It can convert RTF, DOC, DOCX, ODT, COD (the proprietary encrypted document format of Atlantis Word Processor), or TXT documents to the RTF, DOC, DOCX, COD, TXT, or EPUB format.
You can find details here:
The new beta can be downloaded from this page: