ONTRAPORT is strictly permission-based for email, and to remain complaint with the various spam laws we do not have a mechanism to add additional contacts to the database. At least not one that is automated and available via a Smart Form at the current time. But there are a couple of options.
We do have a new feature named Custom Objects, currently in wide beta testing as of November, 2015. It could enable you to have a Smart Form add additional contacts (to a separate database) that are related to the original purchaser purchaser. If you need to be able to email the additional contacts, you can have ONTRAPORT send them an opt-in email message. If they approve receiving email, they could then be added to a reminder email sequence for reminders. Our knowledge base article on the new feature is here: http://support.ontraport.com/entries/... (it's an advanced, technical feature that is available by special request).
An alternative to this is to create several custom fields in the contact database. First name, last name and email address fields would have to be created for each one of the possible add-ons, requiring several fields. This works best when only a few extra tickets are needed. These fields can be exposed on the Smart Form and filled in by the purchaser. Have the Smart Form create a Task with the contact information, and manually view the Contact Record. You could then manually add the contacts in the fields to the Contacts database. If you do this, you must send them the opt-in email first (found in the Actions menu) in order for you to send them email.