You can keep your original .html pages as they are, except for the home page, if you want to move to Wordpress. Wordpress lets you have what's called a 'static home page', which would let you use your original home page content - it would just reside within the Wordpress template layout. Depending on the size of your site, it could be a bit of a job to integrate the current site's navigation methods with Wordpress' menus in a manner that's convenient for users.
you can keep and re-use your Markup (HTML) and CSS. So design and structure do not have to change when you move to wordpress.
Regarding the file extension, you can decide within Wordpress how you want your permalinks to look like. In general, you do not need a file extension and are better off without it anyway (e.g. in case you switch between technologies).
is it ok to keep html or should the pages be named differently
It isn't clear whether you're asking about existing pages that you plan on keeping as-is, or new pages that you'll be building in WordPress.
A built-in feature of WP (or just about any CMS) is that before doing its stuff, it checks whether the requested file physically exists. So if you've got certain hard-coded pages that will remain unchanged, you don't absolutely have to change their URL. People who've got /goodstuff/supergood/amazing.html bookmarked by that name will get there same as always.