You would want to subclass the Newsletter class
In MyNewsletter.php
class MyNewsletter extends Newsletter {
... overload getCMSFields ...
... add $db information ....
}
In _config.php
Object::useCustomClass('Newsletter', 'MyNewsletter')
You will also need to make a hack to core, since it's not set up for this currently: in NewsletterEmailProcess.php, in the next function, find this code:
$e->populateTemplate(array(
'Member' => $member,
'FirstName' => $member->FirstName,
'NameForEmail'=> $nameForEmail
));
And change it to this:
$e->populateTemplate(array(
'Newsletter' => $this->newsletter,
'Member' => $member,
'FirstName' => $member->FirstName,
'NameForEmail'=> $nameForEmail
));
Then, in your Newsletter template file, you can reference
$Newsletter.MyVar, etc