smbpasswd muss als root laufen. Es kann auch sein, dass der Nutzer umbenannt werden muss bevor das smbpasswd was macht.
Hast du die smbpasswd-Commands schon getestet?
Code: smbpasswd(8)
-a
This option specifies that the username following should be added
to the local smbpasswd file, with the new password typed (type
<Enter> for the old password). This option is ignored if the
username following already exists in the smbpasswd file and it is
treated like a regular change password command. Note that the
default passdb backends require the user to already exist in the
system password file (usually /etc/passwd), else the request to add
the user will fail.
This option is only available when running smbpasswd as root.