The best you can do export the NPC, then import them as a slave.
To do that, first go to the same tile as the NPC you want to export and click on their race icon. You can even do it to your current character.
Alternatively you can check your phone's contacts to view any named NPC's you know. In the example I'm doing the latter with Lexi.
Next check the bottom right corner and hit the first button with the rectangle and arrow. That's the export button.
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Now you can head over to the auctioning block in the Slaver Alley.
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Once there, you can approach the stage to view a list of imported characters. Initially it will be empty. Hit import button to bring up a list of saved characters. You can do this multiple times for the same save, each of them having slightly different obedience levels. Then click the appropriate save and hit back.
NOTE: If the character has images copies of them will be made, so don't go crazy rolling for an ultra submissive slave.
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Next you need to win the auction. Hit that gavel next to your to start bidding on the slave.
You can back out if the price gets ridiculous, but I got lucky here.
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And just like that you have an NPC as a slave. They'll have a slave collar on and their
description changed to a generic slave text, but both of those are easy fixes.
