I haven't tried it but I suspect that the easiest way without recoding a bunch of things would be to make a new object that has objBlock as it's parent. The sprite would be just the flat surface that you can stand on and then in the create event for the object put
event_inherited();
instance_create(x,y,objSpikeDown);
So essentially you have the flat block to stand on and a downward spike jutting out the bottom.