You can add prospects to a campaign that is running whenever you want.
Two scenari are possible:
1. Adding prospects to the campaign for the first time
If you add new prospects to a campaign, the emails will start from step 1. The prospects already in the campaign will continue from where they are (can be step 2 or 3 etc.)
The new prospects will receive the email on the next scheduled date. Which means that if you schedule sending emails Monday, Thursday and Friday and you add new prospects on Tuesday, they will receive the email as of Thursday.
2. Adding prospects that have already been in the campaign
You can add prospects that have already been in the campaign before.
If you add former prospects to a campaign, outgoing emails will resume from where they had stopped. When the prospects had completed the campaign at step 3 and you have since then added 2 more steps, you can add them again and they will receive emails 3 and 4 and not resume from step 1.