Yes, you can! For print on demand products not currently featured in our Shopify app, you are able to add them to the published products within the Shopify store admin area as new variants. 

When an order is placed for one of these new variants, the app wouldn't be able to automatically fulfil them, but you would then be able to place those orders using our manual order form

From your customers' perspective, the end result would be exactly the same. Unfortunately, for the time being we wouldn't be able to provide any product previews of those custom options for you, but we are looking to expand our preview generator to cover more products and   this will soon be available outside of Shopify in the near future.