This is easy. Chickens are social animals, so they will learn from the other chickens in their social group. Bring in a
chicken (or ideally, chickens) who roost, and the rest of your chickens will follow her (or their) lead. Alternatively, bring in a new batch of baby chicks and give them access to roostable areas. Somewhere between 2 weeks and 2 months, they
should develop the instinct get off the ground at night, and when they do, your older birds will imitate.
If you're absolutely opposed to any new chickens, it's a little harder, but still probably doable. Create a bunch of different levels of roosting spaces, including some that are barely off the ground. That should make exploration and experimentation really easy for your chickens, and their instinct to get off the ground will almost certainly express itself.