A really, really, REALLY sharp chisel can be used to pierce the steel door skin enough to form the mortise you need for the bolt plate. Do not use a wood chisel unless you never ever intend to use it on wood again or have the means to completely renew the cutting edge. Cutting the opening for a plate in a metal door jamb is another issue since it is probably too thick to yield to a chisel. If the jamb is 1/8 to 1/4 inch thick you can just drill a hole in it to accept the extended bolt. it will be just as strong as a plate (maybe stronger.)