You are right, you are re-writing the key at 0 instead of inserting...
I suggest you build the list of keys in an array first. You can insert items into the array (Array->Push action).
And then clear the list box and populate it from the array values.
EDIT: ignore that! I was right the first time. "Add item at 0" action inserts new item at the top of the list, all other items in the list move down.