UX Personas (not to be confused with marketing personas) are usually created before, as they help uncovering pain points, needs and expectations of users. How to respond to those is the foundation for the app in relation with business objectives.
Creating personas requires research and data. If you don't have this information already available, I don't think you can create true personas during the workshop. You will just collect the current representations these professionals have from their users, which might be correct or not. The closer the professionals are to clients, the better their insights will be (best are customer support officers).
So, depending on the information available and the people that will be there, you can either try to go for true personas, or just brainstorm specific user needs and problems, as a 1st step before defining the app goals.