Ok, so you should simply be able to select all the layers except the Background layer, and Ctrl G to group them. Then drag the group to the little page-with-its-corner-turned-up icon at the bottom of the layers panel, to make a copy of it.
Then go to the Canvas Size dialog, click in one of the top squares of the three-by-three grid, then double the height of the document.
Then, with the Move Tool selected, and the copied group still highlighted, drag all the layers down.