Of course, you'd also have to convert the existing HTML documentation into DocBook and then generate the mdoc and HTML from that. I would understand concern over adding a dependency on a potentially heavy DocBook toolchain in order to generate the HTML. One possible way around this, though, would be to generate the mdoc from the DocBook using docbook2mdoc, and then generate the HTML from the mdoc using mandoc.
Are you volunteering to do that work? -- Laurent