According to the documentation, IsItalic is a read/write property.
Here is a little code sample demonstrating how to apply an italic font to a particular cell:
var wb = new XSSFWorkbook();
var sheet = wb.CreateSheet("Sheet 1");
// Create an italic font
var font = wb.CreateFont();
font.IsItalic = true;
// Create a dedicated cell style using that font
var style = wb.CreateCellStyle();
style.SetFont(font);
var row = sheet.CreateRow(0);
row.CreateCell(0).SetCellValue("Username");
var cell = row.CreateCell(1);
cell.SetCellValue("Email");
// Apply the cellstyle we craeted
cell.CellStyle = style;
using (var fileData = new FileStream(@"G:\scratch\sheet2.xlsx", FileMode.Create))
{
wb.Write(fileData);
}