First You have to pause workflow to insert a new step.
If you are adding a new step in the middle of your workflow, then it will be executed only for the subscribers who have not reached this step yet.
If you are adding a new step as the last step of your workflow, then it will be executed for all subscribers, even those subscribers of the list.
Note: If your workflow is turned OFF while editing, no emails are being sent at that moment. These emails are not sent once you turn on your workflow again.