Convert to wordpress

The answer is simple: install WP and copy the html content to it.
If the website is large, this is a hell of a lot of work. I did the same several years ago.

Just to clarify — there is no way to automatically “change” a website to WP. You have to set up the WP site and manually copy content over. And like Roy said, it can be a TON of work — but generally worth it in the end.