Either via repurposed ST'09 footage (maybe impossible due to the CBS/Paramount split), or with an aged-up Ethan Peck? The series deals with the aftermath of Romulus being destroyed by the Hobus supernova, an event which "threatened the galaxy" and was stopped by Spock who fell into a black hole with Nero. We know Picard will be in uniform for flashback scenes. His historic rescue mission was presumably to Romulus and this show is in full-on kitchen sink mode when it comes to guest stars, sooooo....