$items = Get-ChildItem -Path "master:sitecorecontentWebsitesitesubnode" -Recurse
    $items | ForEach-Object {
                             [Sitecore.Data.Fields.MultilistField]$field = $items.Fields["Keywords"]

                             $items.Editing.BeginEdit()
                             $field.Add("{6D1EACDD-0DE7-4F3D-B55A-2CAE8EBFF3D0}")
                             $items.Editing.EndEdit()
}