We already have a ticket booked for Machu Picchu but would like to add on a hike to Huayna or Huchuy Picchu. Is there any way to add on tickets for either hike during our visit?

It is not possible to buy an individual ticket only for any of the Machu Picchu day hikes (Huayna Picchu, Huchuy Picchu, etc). Tickets for those add-on hikes must be chosen when you are buying your Machu Picchu entrance ticket. If your tour agency bought your Machu Picchu ticket, you need to ask them if they can return it for a refund and buy you another. Machu Picchu tickets are sold by the government and are normally non-refundable and non-transferrable. However, an agency might be able to make that change for you.

If you bought the ticket yourself, you should buy another one that includes the day hike. Then, when you're in Cusco you can go to the Machu Picchu ticket office on Plazoleta Regocijo and try to get a refund for the original ticket. I can't guarantee that they'll give you a refund.