force specific shell environment variable on process using /etc/pam.d/ rules

Question

I am using slim login manager, and it has its own pam rules file: /etc/pam.d/slim.

auth    requisite       pam_nologin.so
auth    required        pam_env.so readenv=1
auth    required        pam_env.so readenv=1 envfile=/etc/default/locale

@include common-auth
@include common-account

session required        pam_limits.so
session required        pam_loginuid.so

@include common-session
@include common-password

it seems that the pam_env.so rules are setting the environment variables for the running process, such as: USER, HOME, PWD, SHELL, ...

Is it possible, after these have been set, to force SHELL to be /bin/sh? I mean, regardless what shell user has specified in /etc/passwd, I want to force to use /bin/sh for slim process.