I think the easiest way is to open your website in the browser, view source and copy it.

Then paste that code into fresh ImpressPages Drag and Drop CMS installation file ip_themes/ip_default/main.php

Read here how to add required files to the head section: http://www.impresspages.org/docs/working-with-themes/create-theme/

Replace main content with:

<?php
php echo $site->generateContent();
?>

Readhow to add other elements: menu, languages list and so on: http://www.impresspages.org/docs/common-tasks/frequently-used-tasks/