Options for hanging a wreath on a metal door include a magnetic wreath holder or a metal over-the-door holder. While it is possible to use a punch, drill a hole and add a screw, the process permanently damages the door.
Magnetic wreath holders are available at Amazon.com. These magnets have a 10-pound capacity and an extendable hook. They work with both holiday and other decorative wreaths. When putting the wreath away, the extendable hook provides the leverage the magnet requires to remove it from the metal doors. Sold in pairs, two of these magnets, stuck back to back on either side of the glass, hold a wreath on a single-pane window.
For heavier wreaths, an over-the-door wreath holder is an option. These metal devices are thin enough that they do not interfere with opening or closing the door. They extend down the surface of the door and provide a small hook for holding the wreath. Simple holders are available in a variety of finishes, such as white, green or brass. Pottery Barn offers a personalized over-the-door holder with an adjustable height for locating different sized wreaths in the correct position. Made of cast brass and stainless steel, it fits over doors up to 3 inches thick.