Speaking in a Reddit AMA, the game's live producer Lathieeshe Thillainathan answered a number of questions about the returning character, who'll be added as post-launch DLC. Thillainathan made clear that, "Aiden can replay the full game and will have a dedicated DLC with Wrench. You don't need to finish the DLC to play as Aiden in the single player story."
Asked how Aiden will slot into the main campaign given that he's a set character - unlike Legion's NPC-recruited protagonists - Thillainathan explained that he'll be treated like any other character, but will have his personality and character written into the main campaign's cutscenes:
"Aiden will be treated as another member of the team. We are writing the full single player story again with [Aiden's] personality. When you replay the main missions again, Aiden will be integrated into all the original cinematics."
Aiden will be added alongside three other playable characters - Wrench from Watch Dogs 2, Darcy, an Assassin, and Mina, a victim of transhuman experiements who has psychic powers.