Single node cluster is not recommend HADR solution. One of the reason I do this is SQL Server AlwaysOn require the “Windows failover Cluster name” for the SQL Server.
Below screenshot from one of the standalone installation. The host does not have failover cluster feature installed, the SQL Server instance property is complete gray out
Add Failover Clustering feature . This would not cause down time on the Windows host and SQL server instance.
Add the Cluster Name and IP address
From SQL Server configuration manger , we can see it found the cluster name.
If you already configure SQL server to listen on any port.
This is effective immediately .
This test prove that we can add Windows Failover clustering after SQL server is installed.