You can always add additional emails to the end of a campaign.
The problem that occurs if adding an email somewhere in the middle is what to do with the folks who have already signed up and have received the 3rd or 4th email. If you were able to add a new email between the 3rd and 4th, the new email would become the new 4th email and the old 4th email would become the new 5th, and so on.
Somewhere along the line, some of your subscribers would miss an email. For this reason, we recommend spending time thinking through your customer communication strategy before configuring your marketing campaigns. Of course, if need be, you can always create a new campaign and assign all new subscribers to the new campaign - while you allow the previous subscribers to play out the series from the prior campaign.