Custom PHP with Codeigniter

I'm a one day old new recruit to Codeigniter and i acknowledge that every day is a learning day so i'll be grateful for any help you may provide.

I have a php application that i want to port to Codeigniter. Would i need to start learning CI or would the same code work. For some part of it anyway, would it be possible to use the source code on individual pages. I understand the templating system in CI as well as the MVC (not very conversant in this, but i have been leaning all i can).