Questions about user entry in ks.cfg file.

[Petro]

I am trying to add a small number of users in a ks.cfg such that they 
are also in the "wheel" group so that

user --name=petro --groups="petro,wheel" 
--password="$1$v4X7IlXv$fPls90.jmAV0Z/z..vZKj0" --iscrypted

<and three more>

Then in the %post section I:
mv /etc/sudoers /etc/sudoers_orig
cat /etc/sudoers_orig | sed 's/# %wheel.*ALL$/%wheel\tALL=(ALL)\tALL/g' 
 > /etc/sudoers
chmod 0440 /etc/sudoers

I've tried this with various shiazit in the groups= bits, but it only 
seems to modify/edit the /etc/passwd (e.g. petro:500:10:...) file, not 
/etc/group and /etc/gshadow.

So ultimately my goal is met--I can add these users such that they can 
use sudo, but it just doesn't seem to be right.

Is this correct behavior? This certainly isn't what the documentation 
would lead me to believe.

Regards,
Petro.

P.S. cat /etc/redhat-release
Red Hat Enterprise Linux Server release 5.1 (Tikanga)