If you need to add more ticket types to an existing event you can easily do this in the Partner Portal. Select the event you wish to manage in the My Events page and select the Dashboard. In the Dashboard navigation select Event Details > Edit Tickets.
A list of your already live tickets are shown and you can manage these here. To add a new ticket type select the Add New Ticket Type button.
If your event has multiple dates you will need to select which date you wish to apply the new ticket type to at the top of the screen.
Now you are able to add your new ticket type. For full instructions of how to add a new ticket type please refer to the Adding New Ticket type question here.