got it!
still the question about can you add isrc and / or text to a file that already exists?
EDIT so to say, not write in while generating the actual file.
to explain my problem: when i finally render a mastered title i have the option to type in all kinds of information to the text part (comments, artist, etc...)
but once it is rendered/generated i can't! :(
is there any program that does this job?