Customer portal has not very clear behaviour on trial subscriptions. The only option customer can use is to cancel subscription. We have manual product activation and it is not good to force customer to cancel subscription and create a new one. Customer even can't pay for subscription and make it active. The same it is impossible to add addons.
1) You could choose the actions allowed by the customer as well as behaviour under Settings> Hosted Page Settings> Customer Portal. The customers can edit their account information, billing and shipping address as well as manage their subscription including upgrade , downgrade and cancelling the subscription.
2) If you have several plans (for example, a trial plan and some paid plans), the customer can add his or her credit card details and change from the trial plan to the paid plan, using the customer portal.
3) You could add recurring addons using the Customer Portal. We are planning to support addition of one time (non recurring) addons via the Customer Portal in the near future but we don’t have a firm ETA on it.