You mean you have a live chain already running and you need to add authorities to the set? You need a specific pallet to manage that. Sessions could be part of the solution. But even if you add sessions, you would still need a pallet that allows adding and removing authorities.
You could write your own simple pallet to do this. Or you could use a complex pre-built thing like pallet staking.
You may find this blog post interesting to cover the important aspects of what you are trying to do https://www.gautamdhameja.com/resilient-poa-network-substrate/