How to edit and create behaivours for already exsisting mobs

Nothing can do Klemen clearly said that MCreator will not gona modify any original minecraft files 

You can search code for ocelot and check if there is code for providing specific food 

Actually, you can change the behavior of pretty much any part of vanilla games using overriding methods, Forge events and/or Java reflections, depending on what are you trying to accomplish, written in an order that you should try them in. (For example, the cat food should be relatively easy to do by overriding methods - if the food is your item, or if not, then there are events you can use).