I am sorry, however it won't be possible to stretch an image in Lightroom.
You may use Photoshop application for the same.
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It's pretty simple in Photoshop or Photoshop Elements.
In Lightroom, I believe you can stretch the image left-to-right by using the scale slider under the Lens Correction panel (or in Lightroom 6.6/Lightroom CC 2015.6 I think it's under Transform, I'm not at home right now so I'm not 100% sure), but I'm not aware of a way in Lightroom to shrink the image top to bottom.
Yes, I mis-spoke, the aspect slider is correct, not the scale slider.