I have managed to achieve what I wanted (referencing this code) but not sure if this is the correct way of doing this on version 7. Any feedback?:
$vm = Get-VM "MyVM"
$vm | New-HardDisk -DiskPath "[Datastore] VM/DISK01_3.vmdk" `
-Persistence IndependentPersistent `
-Controller "SCSI controller 2" `
-OutVariable hd
$diskDevice = $hd.ExtensionData
$diskDeviceBaking = $hd.ExtensionData.backing
$spec = New-Object VMware.Vim.VirtualMachineConfigSpec
$spec.deviceChange = New-Object VMware.Vim.VirtualDeviceConfigSpec
$spec.deviceChange[0].operation = 'edit'
$spec.deviceChange[0].device = New-Object VMware.Vim.VirtualDisk
$spec.deviceChange[0].device = $diskDevice
$spec.DeviceChange[0].device.backing = New-Object VMware.Vim.VirtualDiskFlatVer2BackingInfo
$spec.DeviceChange[0].device.backing = $diskDeviceBaking
$spec.DeviceChange[0].device.Backing.Sharing = "sharingMultiWriter"
Write-Host "`nEnabling Multiwriter flag on on VMDK:" $hd.Name "for VM:" $vm.name
$task = $vm.ExtensionData.ReconfigVM_Task($spec)
$task1 = Get-Task -Id ("Task-$($task.value)")
$task1 | Wait-Task