an easy solution consists in adding the needed characters in the definition of the function tep_output_generated_category_path_fs() ... edit the general.php file in your admin directory and make the suitable changes ... for instance, i was able to add the following special characters @ ! ' - é è & # by modifying the $dir_path variable into :
$dir_path = ereg_replace("[^a-z0-9._@!'-éè]", "", str_replace(" ", "_", str_replace("%20", "_", strtolower($calculated_category_path[$ii][$jj]['text']))));