Actualy before Posting this question I found this question similar to my requirement but I need fileds in service task should be presented for user to fill ,rest of the properties like class name etc are to be prefilled and not to available for users,as i am defining this custom task so user should just drag and drop no need to enter those properties.
So I am not finding exact solution as per my requirement. According to the topic that i have mentioned here I got to know that by editing stencilset_bpmn.json I can add custom task to palette but it is present in source code not in the war file that is generated from source code.