Redmine

Previously, if I wanted to add functionality to redmine, I would go into the source code and add or modify it from there. I am now endeavouring to use plugins instead.

I'm reading through several guides but I'm having trouble finding a method for adding or replacing code. Hooks seem promising but so far seem to be limited to just adding code after the source instead of modifying it. For example: I'm unsure of how to change how the issue edit forms are rendered through a plugin. Additionally there are too few of them. One of the plugins I would like to create would run some code after the creation of a user, but there are no hooks for the user controller.(http://www.redmine.org/projects/redmine/wiki/Hooks_List)

Thank you for any information or resources.