If your workflow is running, you can still add steps to it. The prospects 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 prospects already received the step 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
If a prospect 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 prospect to this workflow and select the needed step to start.