You have to import plaintext passwords. You can't import already encrypted passwords. Plaintext passwords will then encrypt for that Joomla install. If you need a 1 to 1 import you should be doing so directly through your database (e.g. phpmyadmin lets you import data from several different formats).

You'd have to handle the entire user creation process. So yes you'd need to import into Joomla users table as well. An alternative is import through CB > User Management then import passwords into _users in your database directly so you can bypass the encryption process. The import/export is exactly that it's not a backup feature so it will go through the normal user creation processes so you'll need to keep that in mind for future usage.