You can approximate that effect using invisible damaging area objects (just as LPChip explained) or hiding trap objects in a lower layer (e.g, putting a CO enemy sprite on layer 6, a fake CO background at layer 5 and a proximity bomb on layer 4). The problem is that the "damaging area" will be static and won't move with the animated custom object. If you want to do this properly, you'll have to design your level for a modified version of KS that has this feature. I think Purple Pineapple's mod (KS ACO) supports "deadly" COs, but I don't know how it works.