Cast iron, by design, in one of the hardest metals found in the home. That does not mean, however, that it is impossible to drill a hole in it if needed. But the drilling cannot be done quickly with a single drill bit of the desired size. Rather, drilling a hole in cast iron requires that you drill several holes sequentially, gradually increasing the size of each hole until the desired size is reached.
Attach the smallest drill bit to the electric drill.
Put on the gloves and the safety goggles. Drill a small hole in the desired location on the cast iron. Hold the drill steady while applying consistent pressure.
Attach the next drill bit to the drill, and enlarge the hole by drilling in the same point with the larger bit.
Replace the drill bit with the next larger drill bit. Gradually increase the size of the hole with larger and larger bits. Repeat the process until the hole in the cast iron is the size you want.
Do not apply excessive pressure to the drill bit when drilling. Cast iron is a hard enough material that you could break the bit if you apply too much pressure. Be sure to wear safety goggles and heavy gloves when drilling. Cast iron is known to chip, meaning material could fly off and endanger your eyes.