If you want to convert your site to WordPress, you cannot "keep it in HTML format". WordPress pages are dynamically-generated using PHP.
Any HTML site can be converted into a WordPress site. Some are easier to convert than others. 😉
Google "convert HTML to WordPress" and you'll be deluged with tutorials and articles.
Of course, if you just want to display a few static HTML pages within your WordPress site, just add them to a Menu or Menus.