You should be able to do what you originally planned.  Review the following KB article:  emc259251: "Domain options are grayed out when connected to Unisphere Server"
Hello,  I've installed UnisphereServer on my laptop hoping that I will be able to use it to manage multiple domains, but multi-domain option is greyed out. Is it a way to add multiple domains under one umbrella? What do I missing?  Thanks.
Multi-domain management from Unisphere Server 1.2.2.1.0045-1