Yes, it's a shady business, but you don't have to indulge in it, and these people were already slaves to the Star Gods anyway. Will you be a kinder master than them, Cap'n? The Slavery option is available in the Workshop, and it permanently turns a prisoner into a slave. A slave is worth less money than a pristine hostage, but will generate monthly income and storage space through the power of human networking. He or she will also be relieved of their possessions, just like through Robbery. Some people are mentally unfit to be used as slaves - they'd be more trouble than help.