resizing an active layer

[Argos]

I have a minor problem. I have a group photograph that I'd like to frame. Unfortunately the photograph is in different proportions to the frame (It's wider in proportion to the height). In normal circumstances, I could crop to the correct proportion, but that would mean losing the people to the sides of the picture. Framing, as, is leaves an ugly band at top and bottom of the picture. In this case, a simple change is possible and easy, in principle. The foreground of the picture is grass. If I could add a blank area to the bottom of the picture, I could use the clone tool to extend the grass to the new bottom of the picture. It's sufficiently amorphous and out of focus, to add without any distinct addition. I've searched the manual, without success. I've a number of pictures which could be improved by such a solution, or variations of a solution.
Cheers

BTW, this is my first post to this forum, so, apologies in advance, if I've not done it 'by the book'