Hi technically aworkflow which is started, you cannot add a rollback now. But you can add a rollback feature in your worklfow and it will work for new records.
There is Rollback to workflow activity that you can use to rollback to specific activity inside your flow.
And if you want to run this for existing records which are currently in progress, you can write a background script and restart the flow so that the new published WF triggers.
Mark the comment as a correct answer and also helpful if this helps to solve the problem.
