Not realy sure what you mean: the extension export will take all columns used in the hypercube, the app developer will select columns in the property panel.
Same goes for exportData, all columns will be exported. If thats not what you want, you could create a new hypercube, with just the columns you want and export that or loop through the rows and pick just some columns.
Hope this helps
Erik Wetterberg https://extendingqlik.upper88.com/