I have never dealt with chickens that didn't learn how to naturally but, If it were me, I would do this....Mix up some food grade diatomaceous earth and some wood ashes (just wood and old newsprint, mind you, no coated magazine paper or wood that has been painted or treated) and a little traction or even sandbox grade sand like I mentioned mefore. Mix this stuff up in a container big enough for a chicken, like an old rubbermaid, a cardboard box, or a cat littler box bottom works well. I would put one of the hens in and start dusting her yourself like you would if you were treating her for mites. Get the dust up under her wings and everywhere. Maybe she will take to it after you show her a few times by doing this. Try to do it also so the others can see. If you can get one hen to do it herself, the rest may follow her lead. Try to use the boss hen of this little flock if there is one. Leave the container in their run at all times so they can see it and become aquainted with it.
The poor little things, that is so sad. I am happy that they have found what sounds like a loving home with you. I hope they can all come around. Their age actually sounds about right...most owners do not keep battery hens past 3 or so years at the most because their laying average begins to drop at that point.
PS you can also bathe a chicken with water and soap, if they are dirty or caked with sludge or smelly, you may consider trying this. Use baby shampoo or bubble bath and a sponge. Put the hen on a dry towel or in a bathtub and use a soaked sponge, washing in the direction her feathers grow. Obviously don't scrub or you will muss up the feathers she does have. Use a towel in the same way to mostly dry her and a blow dryer on low heat for the rest. After washing, I would dust bathe her as stated above.