Convert XDOC to EPUB

Converting QuarkXPress 3.0 Documents directly to Open eBook Files (.xdoc to .epub conversion) is possible and should be quite straightforward when using the original software, QuarkXpress. It allows users to exports their .xdoc documents to .epub files.

This seems to be your only option to convert .xdoc to .epub, since the .xdoc file type is a proprietary format. You will, most likely, not be able to find any third-party alternative or (online) conversion utility.