If i understant your question correctly, you have a workflow template (change) which has a activity that will be assigned to a user. The user getting the task should be able to add a file directly to that tasks and not to any document/part.
I dont think its straightforward. Workflow tasks may not be having underlying framework (it needs to be vaulted,foldered/iterated, access control etc).
Alternatively you can add as an attachment to the change objects itself. But its not specific to the task. You can create a reference also (URL etc)
