Is it possible to send an email to a person who is NOT a lead in the database by just entering their email address in the 'Email Address' filter of the Smart List step in a Smart Campaign?
No, Smart Lists by design act as filters on your existing database. To send to someone not in your database -- e.g. to send an email to an internal stakeholder for review/follow-up -- you can use the "Send Alert" flow step and define the email address in the "to other emails" text box. This email will not be tracked or appear in reporting.