Supplementation is fine as long as its no more than 10% of the daily diet. Instead of adding shell to feed you should keep it separate so that hens can self regulate if needed. A commercial feed is for an average hen but like people some need a top up and one size does not fit all.
Oyster shell being the optimum supplement but should be kept as a separate ration. My hens have all the food waste including egg shells, meat, fat, veg, salad, nut shells etc. I was told not to do this but as long as its not a huge amount is fine and never have issues. My biggest issue last year was just feeding a commercial mash which was seriously lacking something I read the ingredients and wasnt impressed hence why going back to Hipeak No 2. If birds diets are too high in grains, the phosphorous levels can cause the imbalance. My hens also have seaweed meal mixed into the feeder. As my layers are on pasture and moved on regular basis they have a good peck on the ground, grass and weeds. But I still get the odd - odd egg or soft broken egg.