Hi,
there is a way how to add a switch to a cluster if it is behind a non-CDP capable *CISCO* device. I did it two years ago.
You need to use cluster commander-address and cluster member commands in the member and master switch configs.
I can't remember details how to get the member switch MAC address, but I think there is either the Base MAC address used (use sh ver to get it) or you can get it by sh cluster cand or sh cluster mem on a switch (you can make the member a master temporarily to get the address).
I also remember having an IP address assigned to each switch in the cluster but I don't think it's necessary (but it's a good practice).
Generally: why are you using clusters at all?
My opinion is that the only advantage is the possibility to see several devices in one browser window.
But you can get the same functionality with CiscoView StackMaker without any problems with Cluster Management Protocol using strange IP addresses derived from the switches MAC addresses for internal communication (to identify them use debug cluster .....), these addresses are checked by http ACL, etc.
More: clusters are not supported neither by Ciscoworks nor HP OpenView.
Regards,
Milan