You can avoid the need for making a new document this way:
1. Create a new layer.
2. Use single pixel marquee to make selection.
3. Fill selection with black.
4. Zoom in on this one pixel line and then turn off ALL other layers but this one. (Alt+Click the eye in the Layers pallette) Deselect line.
5. Using the rectangular marquee, select the line and ONE pixel below it. It doesn't have to be very wide and not the whole width.
6. Select Edit>Define Pattern.
7. Clear the layer out. (Ctrl+A, Del)
8. Zoom back out to whole picture and then go Edit>Fill... and use Pattern for the contents.
This will fill the whole image without regard to image width.