Greetings.
I believe it has more to do with the Vsphere license you have installed on your BE7K host.
I have seen instances where you need to take the license key you have installed on the local esxi host,, add it into vcenter licensing,,, then assign from vcenter to the host you are trying to join to vcenter.
See licensing versions: http://www.cisco.com/c/dam/en/us/td/docs/voice_ip_comm/uc_system/virtualization/virtualization-software-requirements.html#license_comparison
http://www.cisco.com/c/dam/en/us/td/docs/voice_ip_comm/uc_system/virtualization/virtualization-software-requirements.html
When you say it cannot be added, is vcenter complaining about the licensing?
There are going to be certain interop requirements for various versions of vcenter and the esxi version your host is running.
Thanks,
Kirk...