In short, it depends on how much you need to edit on the original pages and how many pages there are.
If the content is never going to be edited, then you can maybe create most as just normal masterpages (umbraco templates). You can then just add in Navigations and other CMS spec
Or another way of doing it would be to create 1 template and add a big text box to the DocType which you can then paste the HTML into. However this will mean you will end up with loads of HTML in the DB.
If you want to make it a 'true' Umbraco site and use as much of the CMS functionality as possible then there is no really easy way of converting HTML into an Umbraco site.
If the data was in a DB you could use CMSImport and map the old to the new DocType fields, but you would still probably have to do some HTMLTidy work on it before importing.
Depending on the amount of pages, the best and easiest way is usually to just copy and paste in my opinion because it allows you the chance to perform a content audit at the same time and clean up HTML where necessary.