An epub is just a zipfile full of html files (among other things). Use Sigil to split the html file the way you want it, and then unzip the epub and snag the html files. You may have to fix some links afterward. I have to say, though, that that seems like a very long driveway to a small and rather unimpressive house.
You'll spend a lot of time looking for tools that will "automatically" help you construct an epub by hand.