You are looking to add category attributes. The reason you are not finding them in code is that they are setup in the database.
Create a custom module with a setup script that adds attributes to categories. Once you have set up the attributes, you can use the normal getters and setters to access and update the data.
I could dig out some code, but a quick google search found this blog entry that appears promising for what you want to do: http://www.meteorify.com/2013/03/quick-tip-adding-custom-category-attributes-to-magento/
Or, here is a stack overflow question/answer that might help: Magento 1.7 Add attribute to category