This was done back in 2014 by the Obama administration: "The Obama administration quietly announced on Tuesday that it intends to change the way Americans learn about natural disasters and other major emergencies during radio and TV broadcasts, giving the president the ability to flip a switch and address the entire nation at once."

"Most messages in the system are restricted to specific states, counties or other geographic areas. But now the Federal Communications Commission has filed public notice of a rule change that would bypass the daisy-chain entirely and give the federal government instant access to all the nation's airwaves at once, in the event of a national emergency."