You can try this way :
var doc = new XmlDocument();
doc.Load("path_to_xml_file.xml");
var elements = doc.DocumentElement.SelectNodes("/rootNode/category/string");
foreach (XmlNode element in elements)
{
//check if content of <string> is not CData section
if(!(element.FirstChild is XmlCDataSection))
{
XmlCDataSection cdata = doc.CreateCDataSection(element.InnerText);
//replace inner text with CData section
element.ReplaceChild(cdata, element.FirstChild);
}
}
doc.Save("path_to_xml_file.xml");