Can passdb be bypassed for non-plaintext authentication mechanisms

I am using Active directory authentication via gssapi for most users. In dovecot.conf I have:

auth_mechanisms = plain login gssapi auth_use_winbind = yes

I also have

passdb { driver = shadow } userdb { driver = passwd }

for those few users who are NOT AD users.

Even though the AD users do not exist in /etc/passwd or /etc/shadow, Dovecot ALWAYS first looks them up in shadow, which ALWAYS fails.

The https://wiki2.dovecot.org/PasswordDatabase wiki says, "these databases can't be used with non-plaintext authentication mechanisms."

Is there a way to bypass checking passdb (and userdb?) for these mechanism?

--Mark