No. The plugin can not dynamically add items into the style map. The plugin is actually very, very dumb. The underlying Mammoth python library is what does all of the heavy lifting. The plugin merely makes the user-defined style map available to the mammoth engine and makes sure everything that comes back out can be cobbled together as a valid Sigil EPUB. The plugin wouldn't have the first notion of what kind of styles the DOCX has in it. Mammoth hasn't processed it yet. A dynamically created style map based on the DOCX's defined styles would be putting the cart before a horse that hasn't been born yet.
And I'm not Roger, by the way. I'm Doug.
