To manually add new opportunities to an existing campaign, go to any stage of the workflow of a Campaign: Inspect, Personalize, or Compose. Click on the 'Add Opportunity' button in the top right side of your screen.
Enter the URL if this is a website opportunity:
You can even add email address opportunities to your campaign:
If this button is not appearing within your account, you may have certain priviledges restricted. Ask an Admin to change these priviledges for you in the User settings.