Can you be more specific about your use case? Your initial description sounds like you want to have all of the fields of the user task but pre populate them with essentially a script and store the values. Why do you think you need customized service tasks and modeler changes for your use case?
Right out of the box you could use input variables on a service task to replicate each of the form fields or you could use modeler element templates to provide a more custom form in the modeler. https://docs.camunda.org/manual/7.8/modeler/camunda-modeler/element-templates/
And then you can process your data into a process variable (Json or java object).