Ability to create a new service through Customer Portal

We have alot of B2B Customers and we want our B2B customers to have the ability to add new Repair Services while making the Ticket from the Ticket. Either it should be a text field or they should have the ability of adding a repair service that doesn't exist in the drop down. We don't like the option of adding device issue in the internal comments from the Portal once the Ticket is created.