I did this on an old OpenACS 3.2.5 project. It's limited to a few specialized cases, ugly, and a potential security hole. You may want to check out LDAP authentication instead.
Here's how I did it anyway.
- Synch the unix password and the OACS password first.
- Modify the change password .tcl script in OACS. After changing the password in the database, call a shell script that changes the user unix password.
- Don't allow unix users to change their password from the shell or from some other utility. Have them modify their password through OACS.