Ciao
VMware horizon documentation talks about:
"Users can access remote desktops delivered by machines that are not managed by vCenter Server. These unmanaged machines can include physical computers and virtual machines running on virtualization platforms other than vCenter Server. You must prepare an unmanaged machine to deliver remote desktop access."
I suppose you are talking about other hypervisors like Hyper-V
However, I am not clear about the problems for which you cannot use the vCenter
I have never tried (in the past I have hooked active VMs on VMware Workstation or physical machines) but you can simply try to remove and reinstall the horizon agent on a VM belonging to the vCenter that you disconnected from the Horizon.
If during the installation you see the screen asking you for the connection server to connect the agent to, you are already well underway and the VM should appear in the list of unmanaged VMs.
