According to the ArcGIS Desktop Help, the only way to add personal database logins is through the WFM Administrator:
"If you would like each of your Workflow Manager users to connect to the spatial data with separate logins, choose the Each Workflow Manager user will connect with their own database login option."
However, the responsible administrator needs to enter the username/password credentials.
In general it is not desirable that administrators are handle personal passwords in such a way.
My question: is it possible to set up individual Data Workspace credentials in a more secure way? For example by letting end-users enter their passwords themselves through the WFM application?