you have to know custom php development.
>Create HTML layout as per word press default theme.
>Select your architecture
>then you have integrate that html template into wp theme
>go to theme of wp
>copy the one theme on your HD
>now integrate page by page.
You most likely won't get it to look exactly like your current html site. Best to be clear about that up front. But you can get it close, and because it's an html site, the design could probably use a refresh. I re-create html sites all the time for clients and aside from their sites now being responsive =), they are often pleased with the re-design. Plus it offers a great reason for a marketing push: the re-launching of your site.
Best of luck!