Phonetics are implemented as part of the Rich Text functionality. For more information see Using Rich Text
var wb = new XLWorkbook();
var ws = wb.Worksheets.Add("Using Phonetics");
var cell = ws.Cell(1, 1);
// First we add the text.
cell.RichText.AddText("みんなさんはお元気ですか。").SetFontSize(16);
// And then we add the phonetics
cell.RichText.Phonetics.SetFontSize(8);
cell.RichText.Phonetics.Add("げん", 7, 1);
cell.RichText.Phonetics.Add("き", 8, 1);
wb.SaveAs("UsingPhonetics.xlsx");