foreach (ICellImporter cell in rowImporter.Cells)
{
    string value = cell.Value;
    var format = cell.Format.NumberFormat;

    var cellValueFormat = new CellValueFormat(format);
   
    ICellValue cellValue;
    CellValueFormat valueFormat;

    CellValueFactory.Create(
        value,
        worksheet,
        new CellIndex(0, 0),
        cellValueFormat,
        out cellValue,
        out valueFormat);

    var formattedValue = cellValue.GetResultValueAsString(cellValueFormat);
}