Have not done this in a while, but I believe you have to do it a character at a time. Look up the spec for strings and make sure you set the length or delimiter correctly.
Are your ST9 strings all text and not number values? If so, that is more difficult to convert. I think a program could be written to convert all-text Strings to ASCII A-type file characters (2 charcters per word), but I have not seen a program on this site that does that.