I have a 2-node Windows 2003/SQL 2008 cluster with multiple instances, and I
want to add SSAS to one of the instances. When I run the installer with "Add
feature to an instance", I get to the final rule check and I'm stopped with
"Existing Clustered or cluster-prepared instance" rule failed, and unable to
continue. I've tried removing the second node, but I get the same error
(since the instance is still "cluster-prepared".

How can I add a feature to a clustered instance? This instance is already
production, so uninstalling and reinstalling this instance completely isn't
an option, and I'd prefer not a add a completely new instance just for SSAS.