If you created a campaign and would like to add one specific person to it, you can do it by going to the campaign, pressing on Action, and on Add by profile link button:
In this case, you can add one person using their LinkedIn profile link, without making a whole new search. Just copy the profile URL on Linkedin and paste it in the pop-up window:
Once you click Add the person will immediately appear in your campaign and you can reach out to them.
Keep in mind that you can add only one person at a time, however, you can add multiple people this way.