Using socket commands to add a new server to a backend

[Tugberk Ugurlu]

Hi,
Is it possible to register a new server under a backend? This is similar to 
what is being asked here: 
http://haproxy.formilux.narkive.com/1OibZABp/using-haproxy-socket-to-add-new-servers
I cannot see the command available under the documented list of commands: 
http://cbonte.github.io/haproxy-dconv/configuration-1.5.html#9.2
I am not sure what is the best way to handle rolling out new servers and taking 
down the old ones without any downtime through an automated way.
Thanks,
Tugberk