A good article on supplementing your koi's diet, however, CAUTION - feeding your koi live food is highly discouraged.  Live food has a high probability of bringing viruses and pests into your pond. For this reason, K.O.I. never recommends live food for your koi.  
 
Feeding koi is probably one of the most enjoyable things to do in our hobby. It’s always an amazing sight to see your swimming jewels rushing to greet you at the edge of the pond with their mouths wide open, begging for a handful of food. But what are we providing our precious pets?
 
Beyond Koi Pellets: Expanding your koi’s diet for nutrition and fun - POND Trade Magazine