I guess I'm confused. Converting a template to a cms is very simple.. regardless of where it's from?
To simply change the sidebar as an include, all you really need to know are include() and require(). They're pretty much identical, except for the way they handle errors.
Example:
PHP Code:
include(sidebar.php);
Simple as that, really. Just remember that every .html will need to become .php in order to work. If you want to keep your page ranking and url structure the same, use .htaccess to rewrite .php to .html. You'll keep your .html extension, but the page will actually be parsed with php.