I am starting the conversion to Drupal with a theme conversion. For this, I see two solutions: A Nuke Theme engine, that will run any nuke theme on Drupal. A Custom Conversion of the theme.
I chose the latter, because the cleint wanted to implement some big technical changes in the layout. It will look the same, but the HTML behind it will change a lot. So we need to rewrite quite some parts anyway, which means that its much easier to create new PHPtemplate theme.
I use khtml2png to generate a set of images from the original site. These images will be used in future as Background image, for reference.
Next I saved two pages as HTML (full page) with Firefox.. These plain HTML pages will be used as framework and reference for the page.tpl.php, node.tpl.php, block.tpl.php. Once saved as full all images should already be available.
Now we have all the files available that will be used as reference.