Thanks for your answer. I have it so a question can only appear in one exam, with the domain model a one-to-many, so I would use the pool to disassociate from an existing exam and then associate unassociated questions to a new exam. As someone would only be given a second exam if they fail the first so I don't want to allow the same questions in multiple exams.
I was trying things in the meantime since and found a way which works. Having a datagrid of exams from database, then below it a dataview listening to the exam datagrid. Then within that dataview a datagrid of questions from database.
So I can select an exam then multi-select questions and press a button in the question datagrid which calls my microflows and passes both the exam and question list as paramaters.
One thing that's not clear from your question is whether a given question can appear on more than one exam - if so, that would mean the association between Exam and Question entities would need to many to many.
If that's your domain model, you can do this with a standard Mendix page without the need for a microflow. Create a Master Detail page using the new page wizard. The Master would be a list of Exams, the detail would contain a Reference Set Selector of the questions for the selected Exam. Just be sure to commit changes when the question list is changed for an Exam.
Hope that helps,