AFAIK you cannot create custom reports in vCOPS. In 5.6 release notes there was something like
Custom Reports (Experimental)
This release introduces an experimental feature for configuring user-defined and pre-defined custom reports.
But never saw it in 5.7 release. may be its possible that way by using the XML-based reporting feature to generate capacity optimization reports at the custom group level, and to run custom reports that contain performance and operational data.
But that is it.
Actually you can create custom reports in 5.7 as well.
The reports are based on the Planning views. You can create your custom views and base the new report on them.
The process is not very simple, it involves writing xml files and running shell scripts but it is doable.
Take a look here for more information: https://communities.vmware.com/thread/434623
Hope this helps,
Alex D.