CakePHP to CodeIgniter conversion
CakePHP is newer, more feature rich and heavier than Code Igniter (CI is designed to have a much smaller footprint) so you will most likely find yourself creating functionality in CI to match cake's.
CakePHP also handles some core capabilities differently than CI. For example, routing functionality is handled via a dedicated class rather than per controller as in CI (via _remap).
I have never seen a tool that helps to automate such a conversion as it's rarely, if ever, done.
Taking all that into account, I believe the best approach is to rebuild your application from the ground up, drawing on your existing code and database schema.