Conventional wisdom is that hammers are case hardened (soft inside, extremely hard, thin exterior). However, some brands may be high-carbon, hardened and drawn (tempered) so that they are harder (and tougher) all the way through. I do not know for sure. Nevertheless, they will not be easy to just drill through to produce holes for lightening purposes. In either case, drilling such metal is a problem.
A side note: Most of the people (but not all), who respond to questions like this have no credentials relative to what they are posting, they are just repeating what they have been told or read, that may not be true (Example: "You cannot weld Cast Iron.").