That's a tougher conversion, as HTML parsers are extremely lenient: they don't take the notion of "well-formed" as seriously as XML does.
So, it depends on how much your HTML pushes the bounds of what is legit and what will pass. You might be able to do the job with XSL if the HTML isn't too wild. Before I tried that, though, I'd Google for a tool that might already be out there. Do some research.