Add new field to CSV pointing that newly created users must change their passwords at first login.
It will simply set user preference auth_forcepasswordchange=1 in the database table 'mdl_user_preferences' by calling:

set_user_preference('auth_forcepasswordchange', 1, $user->id);

It is called even now if you 1) leave user password blank and 2) check the flag to create user password (line 534 in admin/uploaduser.php). But it is not exactly what I want - I should wish that password remains the same up to first login of the user.