Ce l'ho fatta!!!
Metto il mio codice, magari può servire a qualcuno :

var XLWB : ExcelWorkbook;
XLWS1 : ExcelWorkSheet;
ExcelApplication1 : TExcelApplication;
sheet2, range : variant;
const
LCID : DWORD = LOCALE_SYSTEM_DEFAULT;

ExcelApplication1 := TExcelApplication.Create(nil);
ExcelApplication1.Connect;
ExcelApplication1.Visible[0] := false;
ExcelApplication1.Workbooks.Open(fileDaAprire, Null, Null, Null, Null, Null, Null, Null,
Null, Null, Null, Null, Null, Null, Null, 0);

XLWB := ExcelApplication1.ActiveWorkbook as ExcelWorkbook;
XLWS1 := ExcelApplication1.Sheets[1] as ExcelWorkSheet;
XLWS1.Activate(LCID);

Sheet2 := ExcelApplication1.ActiveSheet;
sheet2.cells[riga, 2].Borders.LineStyle := xlContinuous;
sheet2.cells[riga, 2].HorizontalAlignment := XlHAlignCenter;

XLWB.Save(LCID);

ExcelApplication1.Workbooks.Close(0);
ExcelApplication1.Quit;
ExcelApplication1.Disconnect;
ExcelApplication1.Free;

Ciaooooo