Add CMS subsystem to Codeigniter site
It depends on what you have already. Do you have RBAC in place? If so, that's already half the battle.There's a lot out there but integrating those might turn out to be more work then writing your own system.
I've answered a similar question a while ago, about merging existing CI projects with a CMS, it might give you some more insight:Port Codeigniter App to PyroCMS