I didnt get your question here. By custom task, do you mean you want to perform some logic ? If so you can use service task. If you mean you do not want to use activiti designer, you can always use eclipse to design the processes. If your question was something else, please explain more.
I want to create a custom task of my own by extending the Task Class. Like we have User Task, Service Task, Script Class etc, I want to create a custom task where I can add my own logic and it becomes available in Activiti Explorer for users to add in their model.