I'll be brief because a picture is worth a thousand words. See this larger page if you need more information.

Basically, just add a folder to your project workspace containing your custom templates, select your model's .eogen file and in the section marked titled "Templates" change the three fields for templates, superclass, and subclass appropriately. Then save and right click your .eogen file and select eogenerate to get freshly generated classes if you are not using the automatic eogen option in Eclipse.