There is option to add any number of Attachments or Comments in In-Progress or Completed workflows 

The attachments or comments option will be visible in the workflow beside the Action History. The add or delete or view permission of attachments/comments will be based on permission set for that task. There are settings available to define any type of attachments/comments or set attachments/comments mandatory for selected tasks.

There is also settings available in Model Definition to set permission to add attachments/comments for completed workflows.