You will lose a part of the picture but it's possible: you just put bars on top and bottom to get to 16:9 ratio.
With Pan/Crop you can "correct" the framing behind these bars so you have what you want the most in the picture; kind of "reframing" before putting those black bars.
Make sure maintain aspect is checked/selected for the clip (s) -> in the pan/crop dialog enter the new height measurements for your desired output -> optionally as futZ wrote, move the cropped area around as nec, probably by changing the location of the crop center -> have project set to full frame size, and render to full frame size and the result should be letterboxed.