Service Now is like Lego, so it's technically possible to build anything. But as the other posts indicate, it's important to understand why you need to add ad hoc tasks. In my experience this is a necessary evil, since early on it's easy to overlook a step or a task. The important thing is that if you're going to allow ad hoc tasks you need to make your workflow a little smarter, or else it will continue on to it's inevitable end without regard to ad hoc tasks that it did not create. If your workflow is closing your CHG record, add a run script action that looks for open tasks related to the current CHG and prevents the workflow from closing if open tasks are found. If you are closing the CHG manually, add a business rule that does the same check.
