Food Processor Feeder Tube Hole - What Is It Called and How Does It Work?
From the cuisinart manual parts description section(emphasis mine):


  
  A small removable pusher that fits into a central tube in a large
  pusher. The small pusher is for processing food like single carrots or
  cucumbers, for continuous feeding of small, hard food like garlic,
  onions and chunks of cheese, and for adding liquid while the machine
  is running. A funnel helps when adding dry ingredients like flour and
  sugar through the small feed tube.
  


So it looks like you are using it correctly. I am going to have to try using this when adding milk or butter milk to my biscuits.