Non-standard xml and powershell

As a result, when I run this command:

$Data = [Xml] (Get-Content "C:\test.nsxml")

I get this error

Cannot convert value "System.Object[]" to type "System.Xml.XmlDocument". Error: "The specified node cannot be 
inserted as the valid child of this node, because the specified node is the wrong type."

The problem is that I can't convert the document to standard xml because it is used by a third party application that needs to have the file in its proprietary format.  Yet I need to manipulate the xml elements in it so I can customize the file.

What can I do?

Thanks!