[Samba] How can I set a user's password to expire in samba 2.2.3a

[vinh_tran at hn.vnn.vn]

Hi all,
Can anyone help me to set an user's password to expire in Samba 2.2.3a
(functioning as PDC) to force them to change password in specified period
of time?
Samba using the local unix password database (passwd file and shadow file)
System: Samba 2.2.3a on RH7 as PDC, 40 users

-----smb.conf----
password = user
unix password sync  = yes
passwd program =/usr/bin/passwd

thank you for any advice