If you have a structure like this:
<Services>
<Service>
<Name>My service</Name>
<Label>Availability</Label>
</Service>
</Services>
and want to add a node <Source>
so that it looks like this:
<Services>
<Service>
<Name>My service</Name>
<Label>Availability</Label>
<Source>
<Composition/>
<ServiceRef>My service ref</ServiceRef>
</Source>
</Service>
</Services>
you could go about it like this:
# load XML file
[xml]$doc = Get-Content "C:\service.xml"
# create node <Composition>
$comp = $doc.CreateNode('element', 'Composition', '')
# create node <ServiceRef>
$sref = $doc.CreateNode('element', 'ServiceRef', '')
$desc = $doc.CreateTextNode('My service ref')
$sref.AppendChild($desc)
# create node <Source> and append child nodes <Composition> and <ServiceRef>
$src = $doc.CreateNode('element', 'Source', '')
$src.AppendChild($comp)
$src.AppendChild($sref)
# append node <Source> to node <Service>
$svc = $doc.SelectSingleNode('//Service')
$svc.AppendChild($src)
# save XML file
$doc.Save("C:\service.xml")
Edit: You can loop over several <Service>
nodes with something like this (simplified):
$doc.SelectNodes('//Service') | % {
$comp = $doc.CreateNode('element', 'Composition', '')
$sref = $doc.CreateNode('element', 'ServiceRef', '')
$src = $doc.CreateNode('element', 'Source', '')
$src.AppendChild($comp)
$src.AppendChild($sref)
$_.AppendChild($src)
}