THE SOLUTION MAYBE IS:
in amavis.conf.in I added the option:
$remove_existing_spam_headers = 0;
Now amavis dont clean previous tag on header of mail.
I change on postfix master.cf file:
127.0.0.1:10025 inet n - n - - smtpd
to
0.0.0.0:10025 inet n - n - - smtpd
Now i can use the reinjection bind directly.
i'll test this schema then post the result here.
BUT, i would like to have alternatives to this, like use a real "read previous tags magical function"

If anyone knows some idea, please let me know.