I don't quite understand Your question, but MODI FILE with XLS file will not work. MODIFY FILE will open the excel file but the user can see some strange charchters.
You can change it to:

m.a=thisform.text2.Value
CLOSE all
USE (m.a)
gcDelimName = ALIAS( ) + '.xls'
gcDelimFile = PUTFILE('excel_file:', gcDelimName, 'xls')
IF EMPTY(gcDelimFile)  && Esc pressed
   CANCEL
ENDIF
*** Let's see if the file is OPEN
fHnd = FOPEN(gcDelimFile,12)
IF fHnd < 0
   ********** File is Open. You can't copy to it
   ******* Put Messagebox() with error
   CANCEL
ENDIF
FCLOSE(fHnd)
COPY TO (gcDelimFile) xl5  && I prefer XL5 instead of XLS

DECLARE INTEGER ShellExecute IN SHELL32.DLL INTEGER nWinHandle,;
                                            STRING cOperation,;
                                            STRING cFileName,;
                                            STRING cParameters,;
                                            STRING cDirectory,;
                                            INTEGER nShowWindow
result = ShellExecute(0, 'Open', gcDelimFile , '', '', 1)
** with the code above You can handle errors returned by ShellExecute

DO CASE
   CASE result ==  0	     && The system is out of memory or resources.
   CASE result ==  2	     && Bad Association (for example, invalid URL)
   CASE result == 29	     && Failure to load application
   CASE result == 30	     && Application is busy 
   CASE result == 31         && No application association
ENDCASE