Originally Posted by Marlin
I would imagine you could drill porcelain with a diamond hole saw.
Imaging and doing are two different things. If this sink is enameled cast iron, the enamel is very thin and very fragile when not in full contact with the cast iron. The surface is quite hard to resist shock from above. Drilling it with any sort of hand drill, even diamond can cause it to shatter because its so thin and once the surface is penetrated, the edge of a bit that is moved or spinning out of balance or true to its own axis will contact the adjoining surface and blow it out. Drilling the cast iron has it own set of problems too. Heat must be controlled or the cast iron will expand or contract faster than the enamel or vice versa and you will get a shatter of the enamel. Just like putting cold water in a hot coffee pot.
Its a very technically challenging thing to do and most pros would even consider it. You run a higher risk of replacing the sink than getting a hole in it unscathed.
If its all porcelain or clay, you might be able to drill it, but I wouldn't risk the sink unless you're willing to replace it.
I'd replace the kitchen faucet to free a hole if possible, or install the dispenser in the counter top.