That does not have anything to do with XSLT.
In some DOM implementations you cannot move nodes from one document to another, you need to use ImportNode or AdoptNode.
So assuming you are using the DOM implementation in .NET try
Code:
oTrans.SelectSingleNode("File/Result").AppendChild(oTrans.ImportNode(oTemp.SelectSingleNode("//"), true))
assuming that oTrans is an XmlDocument, or otherwise
Code:
oTrans.SelectSingleNode("File/Result").AppendChild(oTrans.OwnerDocument.ImportNode(oTemp.SelectSingleNode("//"), true))