This will add a harddisk on a paravirtual controller, and then change the unit number of the harddisk to 0.

$vm = Get-VM -Name VM
$hd = New-HardDisk -VM $vm -CapacityGB 1 -StorageFormat Thin
$ctrl = New-ScsiController -HardDisk $hd -Type Paravirtual
$hd = Get-HardDisk -VM $vm | Where {$_.Name -eq $hd.Name}

$spec = New-Object VMware.Vim.VirtualMachineConfigSpec
$spec.ChangeVersion = $vm.ExtensionData.Config.ChangeVersion
$spec.DeviceChange += New-Object VMware.Vim.VirtualDeviceConfigSpec
$spec.deviceChange[0].device = $hd.ExtensionData
$spec.deviceChange[0].device.UnitNumber = 0
$spec.deviceChange[0].operation = "edit"
$vm.ExtensionData.ReconfigVM($spec)