It's completely possible to create a custom style definition with the same characteristics, i.e. a linked style that is shown in the gallery and will be included as an entry when generating a table of contents, and so on. Mapping it to <h1> when exporting to HTML, however, is not possible.
I could log a feature request for this, but am unsure of the scenario that it is needed for. As a mentioned in my previous reply, those are simply styles with special meta meaning. In fact, I tried creating a custom style in MS Word with the mentioned settings and exporting to HTML just to see if the style will be mapped to a heading in the HTML document, and it doesn't appear to work in that way.
Is modifying the default Heading styles not a feasible option for you? If you could provide a bit more details on the exact needs of your application and the reason you're looking for such customization option, I'll consult with the development team and we will try to propose the best option for you.
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