A topic that some administrators have searched for and not found too much information about, is how to configure a Windows Server 2008 R2 server to be managed from Server Manager on Windows Server 2012 (R2). This is fully possible, and fairly easy. This is a very quick guide to how.
Start by installing .NET 4.0. You can find this here. Then, continue installing Windows Remote Management 3.0 (KB 2506143) from here. After installing these two, configure Windows Remote Management 3.0 to listen for incomming connections with the following command. Remember to run this as administrator.
PS: > winrm quickconfig
Answer yes to all questions. This will automatically configure a listener and firewall exceptions. After this command completes, you will be able to add the server to Server Manager.
If you also require reading performance counters in order to trigger performance alers in Server Manager, you will also need to install KB 2682011.