It would be great to have this feature as in simple useradmin module.
According to man, to get this a shadowLastChange field should be set to 0.
In attachment there is a patch for single user creation
first patch for force user password change
second part of the patch (commit that fixed some typos)
patch for batch operations force password change
added support for force password change during ldap-useradmin batch user creation/modification
Thanks - I will include these in the next Webmin release.