Hello Laser,
If you're appending data at the end of the file, you can just open it, use fx_file_relative_seek to place IO pointer at the end of the file and write data to that file using fx_file_write. Inserting data before the end of the file means that subsequent bytes will need to be shifted further, to accommodate for the additional content before them. Once you do fx_file_open, you can then use fx_directory_information_get to get the file size (alternatively you can just look at the FX_FILE structure). You can either read the exact file size, or more (FileX will do a shorter read), into your desired buffer using fx_file_read. Once you have modified original in RAM. Close the opened file, delete it, and create a new one. Use fx_file_open followed by fx_file_write to populate the file with modified data.
Regards