Can I add all my customers to Brevo, also without opt-in?

Yes. With Sync All Customers enabled, every customer is added to your All Customers List (and the lists of the downloads they bought) after a completed purchase. Only customers who checked the opt-in checkbox are also added to your Default List, and get a consent attribute (e.g. MARKETING_CONSENT) set to yes.

Important: only send marketing campaigns to your Default List, or to a segment of contacts whose consent attribute is set to yes. See Settings.