Intergartion forum with Codeigniter application

Why should you re-invent the wheel when we have so many free open sources available? How about this MyBB as you are looking at it!
By the way, it is absolutely doable by using CodeIgniter. Then you need to come up with system design in terms of database, views and
models / controllers to serve for the system and taking into 'secure' your forum. Admin Dashboard is one of the aspect for your forum
User Access Control so on...

Anyway if it is your learning process then go for it!

Regards