Using the out-of-the-box workflow forms support in Alfresco ECM / Content Services, you cannot add a field to upload documents within a workflow / task form. The default feature set only provides for a form control with which you can select / associate files that have already been uploaded to the Repository.
When I was working for my old employer we did implement a significant addon to Alfresco which added the feature you are looking for to any Alfresco form. It is a customisation that is far from trivial. Unfortunately it is not available as an open source module...