You can pin existing questions within your Dynamic Brief by accessing the question from the Rotating Queue.
Step 1: Open the rotation queue
The "Rotation Queue" is accessible from the sidebar of the Brief Config
Step 2: Edit a Question
Along with changing the text of a question, the "Edit Question" dialog allows you to Pin an existing question.
Step 3: Set the Pinned State
You can use the toggle on the left side of the question editor to toggle between Pinned and Rotating