You can stuff any text into a XML <![CDATA[]]>
section without a problem.
Not very useful though, as you can't use XPath or XSLT on the data in this section.
You need to specify more details in your question - how is the text entered, what format of XML you need to produce? For what use?
Update: (following comment)
Use the XML namespace in .NET - specifically the XmlDocument class - this has a Save method that will allow you to specify a filename to save to (though it is worth looking at the other overloads to it).
You can add elements in a similar fashion to the link you have supplied in your question, using the AppendChild methods.
Something like the following:
XmlDocument xmlDoc = new XmlDocument();
xmlDoc.AppendChild(xmlDoc.CreateElement("form"));
XmlElement nameElement = xmlDoc.CreateElement("name");
nameElement.InnerText = nameCtrl.Text;
xmlDoc.DocumentElement.AppendChild(nameElement);
XmlElement emailElement = xmlDoc.CreateElement("email");
emailElement.InnerText = emailCtrl.Text;
xmlDoc.DocumentElement.AppendChild(emailElement);