One way to accomplish this would be to enable the invite only and must approve users site settings. With those settings enabled, the only people who will be allowed to create accounts on your site are users who are invited to the site. Invites will only be able to be sent by staff users because the must approve users site setting prevents non-staff users from sending invites.
You could then pull email addresses from your sql database and add those email addresses to a CSV file. That file could be used to bulk invite users by following the steps outlined here: Sending Bulk User Invites.