AddRebecca

# If the id 103 does not exist in the XML file, create it as the key, and the firstName value
$xmlFile = "c:\temp\students.xml"
$xml = [xml] (get-content $xmlFile)
$id = "103"

$found = ($xml.students.student | Where-Object {$_.id -eq $id})
if ($found -eq $null) {
    $newStudentXml = $xml.students.AppendChild($xml.CreateElement("student"))
    $newStudentXml.SetAttribute("id",$id)
    $newStudentXml.SetAttribute("firstName","Rebecca")
}
$xml.Save($xmlFile);