Sure, just send in a new XData property to imshow:
grayImage = imread('moon.tif');
[rows columns numberOfColorChannels] = size(grayImage);
subplot(2, 1, 1);
imshow(grayImage);
newWidth = [1 5 * columns]
subplot(2, 1, 2);
imshow(grayImage, 'XData', newWidth);
If you want to change the actual image itself, rather than just how it appears during display, then you can use imresize()
stretchedImage = imresize(grayImage, [rows newWidth]);
