If your workflow or campaign is running, you can still add steps to it. The prospects in the workflow/campaign will receive the new steps if these are added after the latest one they've received.
Example:
If you add a new step in the first position and some prospects already received steps 1 and 2 they will not get this new step.
If you add a new step in the 3rd position, prospects that only received steps 1 and 2 will get this new step. But not the prospects that already received the 4th step.
New steps for prospects who exited the workflow/campaign
If a prospect 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 prospect to this workflow or campaign and select the needed step to start.