How to migrate user passwords from D6 to D7 site using salt module?
It is not possible - both Drupal version stores user passwords in a way that was considered pretty secure when the major version was released. But these ways are incompatible. And by design you cannot recover passwords - it's a one way algorithm. Well, there are ways to hack passwords from D6 now (that's why it was changed in D7) but they are CPU-intensive, and in some countries attempts are outright illegal.

The best you can do is to use _user_mail_notify($op, $account, $language = NULL) - Set $op to 'password_reset' and call it inside foreach over all accounts after migration. That way all users will get an email saying they need to reset their password, with one-time link to do that. Probably there is already a module for that, too, but I couldn't find it.