Hello Eliza,
It's definitely something that you can add to. The values may have been created when the dropdown was added to the table. When creating a dropdown for a table, you either specify to create a new table or join to an existing one. You have two options.
1. You can do an import update to the table. See the Help Center on importing article for more guidance. To find what table to import into, go to Customize -> Tables/Fields and click 'Opportunity'. The fields for the table will appear. Hover over Campaign type to see what table the field is based on. The picture below has the Customer dropdown selected.
2. You may have a custom screen that was added to manage the Campaign Types. I'm not sure if this is true for your account.
Greg