Can I extend ExpressionEngine with Codelgniter?

[eluser]bottleboot[/eluser]
I have experience with this.

Let me tell you this EE is, in my personal view, very limited for expanding on with CI.
I work faster with CI making custom apps. EE has always been working against me.
It's difficult in many cases to add on because there are no hooks and you have to do some very intrusive changes.

No I would not recommend it.