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.
Send Alert - Marketo Docs - Product Docs
Retrieving data ...