Beauty in Design have released a Joomla themes tutorial in taking an existing static template and making it work with Joomla.
BiD have released this tutorial due to the number of requests they've had with customers wanting to know hot to move to Joomla but keep the look and feel of their existing website.
The tutorial covers organising the file structure for Joomla, insertion of the Joomla PHP tags, matching and re-naming CSS conventions for compatibility with Joomla and lots more!
Following this series of tutorials will give you the skills necessary to migrate your existing template and make it work with Joomla.
You can sign up for the BiD tutorial here.