Yes, it is possible to add one email address to an already sent campaign. The tracking statistics and delivery statistics reports allow access to details of the campaign like the recipient list, where you can add one email address. To do this, follow the steps below.
Navigate to Analytics. Click "Campaigns" from the left menu. To do this, Select Analytics> Campaigns tab> Delivery Statistics. Locate the campaign and click the mailing ID. Once the campaign details are open, click the "Advanced" tab on the left menu. Enter an email address in the "Add recipient to mailing" box and then click "Add". A pop-up will then display notifying you that recipient was added to the mailing.
The email address added to the mailing will not be included in the sent reports for the original campaign to the segment or category. However, a new mailing ID with "test of (Title of original mailing)" will be seen in a tracking statistics or delivery statistics report view. The segment column will show no segment name.
If you have more than one email address to add to a campaign, we suggest creating a new segment or category that includes those email addresses. Clone the original campaign sent and update the clone to use the new segment.