the file is created with the -s option:
Code:
fsbext -l -s output.dat originalfile.fsb
and then
Code:
fsbext -s output.dat -d c:\input_folder newfile.fsb
remember that for rebuilding the archive you must have not used the -a option during the extraction because it adds some headers for the reading of the files by the user while the fsb archive wants the raw headerless files as in the original one