To add cars or items to the donation system in Redage V3, you typically need to have access to the server files or access to the game administration panel. You can usually add cars or items by editing the server files, such as the database where the donation items are stored, or through the administration panel where you can manage donations and items. It's recommended to consult the official documentation or the server owner for detailed instructions on how to add cars or items to the donation system in Redage V3.