I found this option .. Scribus *.sla to XHTML
https://wiki.scribus.net/canvas/Scribus_xhtml_using_Scripterbut I have not had an opportunity yet to experiment with it
to pipe xhtml into Sigil. But I say again that Markdown Preview Enhanced in Atom
allows eBook output from Preview Window (right click) or indeed custom output using Pandoc.
There should be no need to maintain two different repositories and workflows for output to Scribus and ePub (and optionally presentations and websites). The content remains common, the outline engines differ. Think of Scribus as able to work in two modes:
(a) Self contained content editor plus layout engine plus scripter (import your content and apply styles)
(b) Layout engine only in a toolchain (push your content into Scribus DOM using scripting). Here we drive Scribus from say Atom via Scribus command line option.
There is the matter of extracting styles from legacy documents (*.odt) but that is work in progress. Remember that since markdown preview is in fact HTML you can embed HTML tags (such as div, style) inside markdown to add styles.
For my own purposes I wrap markdown text and images blocks with <div></div> tags as pseudo textframes and imageframes and renderframes to introduce fonts, css to be piped into Scribus.
In Atom I create a library of style snippets so that styles can be easily added into markdown source pages.