add functionality to some controller

Is there any chance to add functionality to categories controller(for example) without editing main controller file. I mean that it is possible to do addon which will add some functionality?

In my changes, just use:


As an example of adding “stuff” to the checkout controller after the main controller is done but before the template is loaded.

you can use checkout.pre.php if you want to add “stuff” before the main controller is run.