You can't just add 4 corners to a hexagon and have a square because if you orient the hexagon so that you have flat sides on top and bottom, the measurement from side to side (point to point) is not the same as the vertical measurement. Triangles that are 30-60-90 degrees could be added to turn the hexagon into a rectangle, but getting the angles just right when you sew them on would be difficult. I like tesspug's suggestion of adding more hexagons in solid colors.