I don't know how to sync it that way, but you can set it up the other way by modifying your /etc/samba/smb.conf file with these entries (customise them for your system) with Samba 3.0.30:
smb passwd file = /etc/samba/smbpasswd
unix password sync = yes
passwd program = /usr/bin/passwd %u
passwd chat = *New*password* %n\n *new*password* %n\n *Password*changed*
With this method, when the user uses smbpasswd, the passwd program is also called.