Just go ahead and add him to the crewlist just like any other. He was probably removed from the crewlist after he boarded, and while he couldn't be kicked off. Or he's an NPC someone created on there, or brought aboard in a party.
If you try hailing him, you'll find out if he's a PC or NPC.
"May the Grace of Ara go with you, and His Vengeance be wrought upon your enemies."