Request for enhancement. After submitting bug 174204 (system-config-users doesn't use login.defs settings) and finding that this is a normal problem and works as is, it would be a good idea to either modify the adduser program to use /etc/libuser.conf instead of /etc/login.defs, so that automated scripts or CLI addition of users won't miss any congiguration changes done in /etc/libuser.conf. Maybe adduser can be modified to use the same formatted file as libuser.conf and then a simlink can be used. There should only be one congiguration file for simplicity.
No, that would cause infinite recursion :) libuser already (since 0.54.1) imports some settings from /etc/login.defs and /etc/default/useradd. Please sese (man libuser.conf) for details.