I assume, you use an orchestration to call the .NET component.
In the orchestration create a variables: varXmlDocument of type System.Xml.XmlDocument, varXmlString of type System.String.
In an expression shape add:
// Get an XML string
varXmlString = Call.your.Net.component.here()
In a message assignment shape add:
// Create an XML document
varXmlDocument = new System.Xml.XmlDocument();
varXmlDocument.LoadXml(varXmlString);
// Set notification message body
msgNotification.Body = varXmlDocument;
I reckon, you will also add a decide shape to check your varXmlString includes an error description.
Regards,