is it possible to modify previously written data ?

hi,
I read http://www.keil.com/forum/docs/thread14845.asp .
It is clear that, there is no way to modify previously written data (SDCard) with FlashFS.
e.g (as Johann wrote)

fH = fopen(FILE_NAME_CLOCK, "a");
fseek(fH,20,SEEK_SET);
fwrite(jbuf,1,sizeof(jbuf),fH);
fclose(fH);

But I really need to know that, is there any way to implement something like this with low level APIs, instead of rewriting data to a temp file and exchange it with old one?

Thanks.