Sorry I don't fully get the question. If you can please give maybe example to understand?
For workitem in their configuration(.wid) there is a category element and if defined Designer puts it in stencil set section with that name (creates new one if it does not exist already).
Thanks Tihomir , you are right , today i test it and add category in the .wid file and then i can see the custom category with its sub service tasks in the jbpm-designer . Sorry for the not detaied info .