i love to feed my chicks from my hand but when they start getting bigger it does start to hurt lol. The only think i could suggest is to maybe wear a glove. I dont think you can teach them to eat gentler. Maybe someone else has a good suggestion.
I don't think they will peck any more gently than they are. My chickens also peck quite hard but even if it is just one-on-one. You might just use a cap from a juice container and palm it so they get the impression that they are eating from your hand.
I have one that's gentler than the rest, but they all come over to see what she's getting so that doesn't help. The feeding frenzy comes over them and it's all about who can get the most first. I've started just letting them take one or two sunflower seeds from my hands and then I scatter the rest - that way they still know I'm the vending machine, but I don't wind up with holes in my palms!