When adding additional fields to a DW/DSA table we drag and drop them to the table from the Data movement pane on the right. When you drop the field, the data movement pane scrolls to the top (for big or multiple tables). It would be a little bit easier and more intuitive to just right-click the field and select "Add to as a new field". And not scroll to the top so you can continue adding multiple fields in a row.
Simple right-click "Add as a new field" in data movement pane