This will enable vmotion on an existing vmkernel portgroup for all hosts in a cluster.
If you want to create the port group first, check this post > http://www.pcli.me/?p=16

$ClusterName = “MyCluster”
$MyPortGroup = “MyPortGroup”
get-cluster $ClusterName | Get-VMHost | Get-VMHostNetworkAdapter -VMKernel | where {$_.PortGroupName -eq $MyPortGroup} | Set-VMHostNetworkAdapter -VMotionEnabled $true -Confirm:$false

This runs slow and you cannot use a “-RunAsync” to make it faster.

Vmware Link:
http://www.vmware.com/support/developer/windowstoolkit/wintk40u1/html/Set-VMHostNetworkAdapter.html