Introducing new ducklings

We bought our son two ducks last year and much to our surprise they are still alive. We live in southwest Iowa. They bring us much enjoyment and entertainment, they roam the farm freely and interact with the cats and all the other animals and will come right up to us and they let us pick them up to hold. We just purchased 3 new ducklings to add to the others. The older two seemed excited to see them in their box when we first brought them home but the next day when we brought them out in the yard one of the older ones jumped in the box and started pecking at the new ones and did not seem very happy. Will we be able to intermix the 1 year olds with the new ones? Any suggestions would be greatly appriciated.