...
 string currencyFormatString = "";
                switch (currency.CurrencyCode)
                {
                    case "RUR":
                        {
                            currencyFormatString = "# ##0,00р.";
                            break;
                        }
                    case "USD":
                        {
                            currencyFormatString = "$# ##0,00";
                            break;
                        }
                    case "EUR":
                        {
                            currencyFormatString = "€# ##0,00";
                            break;
                        }
                }
...
GetCell(ws, "F", currentRowIndex).Value = someValue;
GetCell(ws, "F", currentRowIndex).Style.Locked = true;
GetCell(ws, "F", currentRowIndex).Style.Numberformat.Format = currencyFormatString;  
...