For the future, because these would have little use now. Also this might be too big a feature
This could happen when fighting a bounty NPC. Fairly rare, not all the time. If the NPC you beat is within your level range but not 25% higher level than you, they might offer to join you. They wouldn’t become a deckhand, but you could place them as either a captain of a ship in your player fleet or a special guard in your clan base
If recruiting scumbags with 300 counts of murder is kinda weird, maybe create a separate board of duelists, and they’d be recruitable once beaten
These wouldn’t be fleet commanders, these guys you could set as the captains of individual ships. Unless that’s what a fleet commander is, in which then these guys are specifically for your base only. If they are set as ship captains, the level of the crew will be based on their level
They would level up either by feeding them gems like the crew guys, or by “training” them, where they would spawn at an empty criminal spot and you’d fight, and they’d level up if you beat them because they learned more about fighting from you. Not as much as rivals. Also they can’t be 25% higher level than you to prevent cheeses from making another Adkins type scenario.
Unnecessary side feature: if you’re in the assassin syndicate or grand navy, you can make a recommendation for your NPC. This would allow them to replace the assassin or lieutenant that’s in the base. They’d have to be the level of the NPC they’re replacing (iirc, 130 for bronze sea), with a range of maybe 15 levels up or down. The player with the highest renown will get their recomended NPC into the slot, though if someone kills him, he goes to the bottom of the queue and the next guy goes in. Possible renown requirement to do this so some marine can’t recommend his buddy for a lieutenant’s job.