Two approaches...
Select (highlight) the clips to be stretched, click on the FX icon in any one of those clips, then click on the green + sign.
Choose LETTERBOX in the Aspect Ratio section and specify a 16x9 ratio in the drop-down configuration box.
Click the sixth icon from the left on the toolbar (replace in chain) to apply this to all selected clips.
Or, during export, choose a 16x9 output format from the Resolution drop-down list.