Hi,
Just to inform that I made it to work, after a few months I tried again with more research and it happens that there is a line in the file "10-auth.conf" in Debian 13, and in this file there's a line which points to another file: "!include auth-system.conf.ext" and in this file there is the following sentence:
passdb pam { } userdb passwd { }
Since the file nsswitch is configured to search the users from Winbind's Samba's base, Dovecot is looking for the users and ignoring the ldap override fields. So I just commented the line "!include auth-system.conf.ext" and now Dovecot uses every field I put in LDAP userdb field and does not use /etc/nsswitch.conf.
I also had to make a few adjustments in postfix, but it's in the manual (LDA/Postfix).
I don't know if I was clear enough but that's it.