Hi Luis,
Yes you can add the second node first. let it sync up, and when the cluster is fully online, go on the leader node and do the CLI command shutdown.
It will then tell the follower it will shutdown, and the follower will become leader.
If you want after that to finish with only one node, then don't forget to remove the other node from the node list in the cluster menu, otherwise it will keep asking and checking where the other node is.
regards
Martin