If your workflow or campaign is running, you can still add steps to it. The contacts in the workflow/campaign will receive the new steps if these are added after the latest one they've received.
If you add a new step in the first position and some contacts already received steps 1 and 2 they will not get this new step.
If you add a new step in the 3rd position, contacts that only received steps 1 and 2 will get this new step. But not the contacts that already received the 4th step.
New steps for contacts who exited the workflow/campaign
If a contact is no longer in the workflow/campaign at the time you add a new step (e.g., they received all the steps of the workflow), they will not automatically receive this new step.
However, you can manually re-add the contact to this workflow or campaign and select the needed step to start.