If your workflow is running, you can still add steps to it. The contacts in the workflow 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
If a contact is no longer in the workflow 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 and select the needed step to start.