No, it's not possible.
vCenter Server 6.7 can manage ESXi version 6.0 or 6.5 hosts in the same cluster with ESXi 6.7 hosts. vCenter Server 6.7 cannot manage ESXi 5.5 or earlier hosts.
vCenter Server Upgrade Compatibility
Diego Oliveira
LinkedIn: http://www.linkedin.com/in/dcodiego
