Any ADvendio object can be extended with custom fields that your business might find useful. Navigate to the Setup and then select the Object Manager and the respective object. If you need more help, please check out the standard Salesforce documentation and Trailheads. Things to consider:
If you use a unique field on the Campaign Item please manually empty it before using the following functions:
Copy Item function in Media Configuration
Media Campaign Clone (you can build your own function by using 8.2.2 Use Cases for the Generic Clone Feature).