i think flax is great! you can overfeed it though, as they can only absorb so much. flax is particularly helpful during the winter and will help keep them well feathered for good insulation. you can try mixing it with their current feed (i would go with maybe 1 part flax to 8 parts feed), throwing it out as scratch (with the added benefit that it might sprout, which would give them added greens), or cook it for a warm treat (boil the water, add the grains-you can do a mixture or just flax- turn off the heat and let it sit about an hour or until the water is absorbed).
as far as i know, as long as they are used to getting any kind of scratch or whole grain/cracked corn, they should have no problem with the flaxseeds.