Customize PAM configuration

[Jone]

Hello! I want enter user/root password only once per session. To do this,
it will probably be convenient to export the password to shell variable.
For example, adding this to PAM configuration file:

   auth sufficient pam_exec.so expose_authtok /path/to/script.sh

But how to write it in system-config.scm? Sorry, I couldn't find any examples.