You are able to add on any of the additional items on to your ticket order after the initial purchase.
You must go back to the event page and select the product you would like to check out, and then proceed through the checkout process as you did when you bought your ticket. When you reach the checkout page, you MUST use the same email you used when buying your ticket (this can be found on the right hand side of your receipt email) in order for it to be linked to your ticket.
These will be available until the 10th of May.