Yes, it's not a good idea to mix it in with their feed. Oyster shell offered free choice from its own container is the best way for a hen to satisfy her calcium needs. Mixing it into the food is much too random, and can easily result in either too much calcium in a hen's system or a calcium deficiency because she's unable to satisfy her own individual need.
A hen will crave calcium when she needs it, and she will be attracted to the oyster shell to satisfy her need.