Error: "The node to be inserted is from a different document context"
You need to import the node into the document before appending it:
XmlNode oNode = moDoc.CreateNode(sNodeType, sName, sNamespaceURI);
//necessary for crossing XmlDocument contexts
XmlNode importNode = oParent.OwnerDocument.ImportNode(oNode, true);
oParent.AppendChild(importNode);
return oNode;