Modifying the output of program RFFOM100

Hi,

I tried using a custom program to run the payment advice transaction.However that is not working as I need to use the std program RFFOM100.

Now I need to modify the file which is downloaded thorugh this std program RFFOM100.

Can someone please let me know is there any user exit I need to use or what would be the best approach.

I dont think making a copy of the program will be a good option.