Unable to add user using useradd

If you include LDAP in the /etc/nsswitch.conf (for example passwd: files ldap or passwd:compat passwd_compat:ldap, etc.) it is enough for getent passwd and thus for useradd to see LDAP entries.

Both utilities have nothing to do with PAM, and happily recognize LDAP entries even if PAM knows nothing about LDAP.

By the way, if you have nscd running, remember to restart it after you change nsswitch.conf.