GIMP :: Moving Selection Contents

I am using GIMP 2.8 on a MacBookPro with OSX 10.8.

I want to cut and move a selection (and, if at all possible, preserve and then enlarge the selection rectangle to perform a subsequent cut and move operation.)

I've got a graphic, let's say it's a 10x10 pixel square in black. What I want to do is to add a vertical column of white pixels in between every existing column of the image.

What I thought to do is the following: first, enlarge the canvas to twice the width of the image, so 20Wx10T (original image on left, blank extension on right). Now select the rightmost column of pixels and move the contents of the selection one pixel to the right. Extend the left edge of the selection rectangle 2 pixels to the left, to include the newly "blank" column and the next column of the image. Move new selection and contents one pixel to the right, and so on, until the expanded canvas is filled and there are 'blank' columns between every column of pixels in the original image.

However, I am unable to find a way to move the selection and its contents. I've tried every combination of secret handshake I can think of, but none work. I'm able to move the selection and essentially duplicate it in another location, but not by cutting it and moving it somewhere else, leaving the initially selected area blank.