Add New value to XML via powershell

Here's how to do it (but you'll need to change your input to be valid XML, 'xmlns' has to be lowercase).

PS C:\temp> cat t.xml
<?xml version="1.0" encoding="utf-8"?>
<gl:GLOBALLISTS xmlns:gl="http://schemas.microsoft.com/VisualStudio/2005/workitemtracking/globallists">
<GLOBALLIST name="ListOfVersions - PWCC">
<LISTITEM value="1.1.3.2705" />
<LISTITEM value="1.1.4.2801" />
<LISTITEM value="1.1.4.2802" />
<LISTITEM value="1.1.4.2803" />
<LISTITEM value="1.1.4.2804" />
<LISTITEM value="1.1.5.2901" />
<LISTITEM value="1.1.5.2902" />
<LISTITEM value="Unknown" />
</GLOBALLIST>
</gl:GLOBALLISTS>
PS C:\temp> [xml]$data = [xml](get-content c:\temp\t.xml)
PS C:\temp> $item = $data.CreateElement('LISTITEM')
PS C:\temp> $item.SetAttribute('value', "1.1.5.2903")
PS C:\temp> [void]$data.GLOBALLISTS.GLOBALLIST.appendChild($item)
PS C:\temp> $data.save('c:\temp\t2.xml')
PS C:\temp> cat t2.xml
<?xml version="1.0" encoding="utf-8"?>
<gl:GLOBALLISTS xmlns:gl="http://schemas.microsoft.com/VisualStudio/2005/workitemtracking/globallists">
  <GLOBALLIST name="ListOfVersions - PWCC">
    <LISTITEM value="1.1.3.2705" />
    <LISTITEM value="1.1.4.2801" />
    <LISTITEM value="1.1.4.2802" />
    <LISTITEM value="1.1.4.2803" />
    <LISTITEM value="1.1.4.2804" />
    <LISTITEM value="1.1.5.2901" />
    <LISTITEM value="1.1.5.2902" />
    <LISTITEM value="Unknown" />
    <LISTITEM value="1.1.5.2903" />
  </GLOBALLIST>
</gl:GLOBALLISTS>

If you have multiple GLOBALLIST items you'll need to select the appropriate one using XPath:

$node = $data.SelectSingleNode("//GLOBALLIST[@name='ListOfVersions - PWCC']")
$node.appendChild($item)

Or you can match part of the attribute:

$node = $data.SelectSingleNode("//GLOBALLIST[contains(@name, 'ListOfVersions']")