You can give them milk in a pan or anything they can't tip over. Chickens love milk, and it's good for them.
But don't panic. Hens sometiumes lay ashell-less egg, it's just a fluke that happens now and then. Unless it's happening often, I wouldn't worry about it.
The milk's still a great food for them, high protein as well as calcium and phosphorus. Especially since you have excess.
If you eat your excess roos, here's something you may want to try, from one of the other members:
Feeding whey to chickens as they grow is supposed to help keep the meat tender, so that if you have non-meat breed roos you want to eat later, they won't be as tough by the time they get big enough to eat. You could make cottage cheese, feed that to your hens, and freeze the whey to feed to freezer-bound birds when you have them.