Adding columns is basically done by creating and selecting Calculated members/measures into the Columns selection area.

In this case, the requirement is to add an empty column to discovery, in the measures area (For cosmetic purposes)

1.  Create a formula with the word "null" in it and save it as a measure (in the context menu).
    

   

2. Add that formula to the discovery in the Value selection area, and make sure "Show Empties" is turned on.