Currently Word-to-Markua is a one-way process, and it isn't perfect or fully-automated.
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When an author wants us to do this for them, we start by using Pandoc to convert Word to Markdown, and then we make the relevant changes to produce a Markua document. In theory this one-way process could be fully-automated. In fact, we've built a self-serve Import from Word feature, but it sometimes errors on real-world Word documents. Ideally we would have Word, EPUB and HTML import features which always worked, and but until that's the case, we have the Convert to Markua paid process which costs a bit of money ($149), since we have to do a bit of manual work for now. (I'd rather have Word, EPUB and HTML import that was flawless, of course, instead of the small amount of Convert to Markua revenue.)

It would also be possible (in theory) to export a Word .docx file from a Markua document on Leanpub. Pandoc can output docx, and internally we use Pandoc to process Markua documents. However, we have not built this in Leanpub yet. Also, even if we did build this, it would not be "roundtrip engineering", since Word documents can store all kinds of layout information and other presentational information which does not exist in Markua, and since Markua stores other types of metadata which do not have an equivalent in Word.
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So, in short, the Word-to-Markua process should be thought of as a one-way process that should only be done once (when the book is done) and which currently involves a bit of manual work (whether we do it or whether the author or publisher does it themselves). It could be fully automated in future, but we're not there yet.