Essentials packages cannot be combined with other licenses. They are sold "as is".
from the EULA at http://www.vmware.com/download/eula/esx_esxi_eula.html
c) VMware vSphere Essentials and VMware vSphere Essentials Plus
If you have licensed VMware vSphere Essentials or VMware vSphere Essentials Plus, the following additional license terms apply:
VMware licenses VMware vSphere Essentials or VMware vSphere Essentials Plus editions (collectively "Editions") to you solely for managing up to three (3) server hosts and your use of these Editions is limited to servers with up to two processors. For these Editions, the server hosts must be managed by the VMware vCenter Server that is provided with these Editions, and that same VMware vCenter Server cannot be used to manage other server hosts not included with these Editions.
André