WASHINGTON — With many Americans still foiled in their efforts to buy insurance under the new health care law, the Obama administration moved Thursday to give them more time to sign up and pay premiums, and it extended a program for people with cancer, heart disease and other serious illnesses. The administration strongly encouraged insurance companies to let new subscribers go outside their networks of doctors and hospitals and refill prescriptions for drugs not on their lists of approved medications. Kathleen Sebelius, the secretary of health and human services, said these steps would ensure that consumers have “a smooth transition...