Add Secondary Node to existing WSFC Cluster
I have Windows Server Failover Cluster setup on 2012 R2 with SQL Server 2012, Can I have my new server with Windows Server 2012 R2 with SQL Server 2016?
  


Yes, you can... however, it will not be readable as a secondary and if you ever fail over to it, the SQL Server 2012 instance will no longer synchronize and your databases will be upgraded. Unless you're planning on doing an upgrade, I wouldn't mix and match versions - it gets you nothing.


  
  Can I add it on my existing AlwaysOn Availability Group, which is configured with SQL Server 2012?
  


Yes, but do see above for restrictions and limitations. I would only do this if you're starting a rolling upgrade scenario and plan to finish it fairly quickly. Do not leave multiple versions in the same topology for any appreciable amount of time.