Populate "Insert merge field" from dataset

[Vitall]

I need to know how can we populate(here "Column name") in "Insert merge field in Editor with the help of Dataset. For reference i put my code here to give you an idea;.....need quick response,thanks

ServiceReference1.LetterServiceClient letterservice = new ServiceReference1.LetterServiceClient();
            DataSet ds = letterservice.ExecuteTagSchema("SearchPatient");
            List<string> listTemp = new List<string>();

            foreach (DataColumn item in ds.Tables[0].Columns)
            {
                listTemp.Add(item.ColumnName);
            }
            this.DataContext = listTemp;
            this.radRichTextBox.Document.MailMergeDataSource.ItemsSource = listTemp;
            this.radRichTextBox.DocumentChanged += editor_DocumentChanged;
        }