It should appear as the option 'Leash', when you select a slave in your cell. Then choose to 'Walk Around' and it should let you lead you slave anywhere in the prison. You may have to Force or Beat Up your slave to get them to agree depending on how obedient they are, but I've been able to take slaves I've had as little as one day to be milked assuming that they're lactating.
Does anyone know how to mind break a slave? I tried searching and didn't find anything. The best I'm able to do is get them to submit to me or confess their Stockholm love for me.
EDIT: Figured it out, just punish them severely over and over.
There are settings to 'weight' randomly generated NPCs towards certain characteristics, but the game is designed to ensure all still can appear so quests that require certain characteristics can still be completed
You adjust the gender distribution of the npcs. But, don't think you can set how the average body parts look. So you can't make everybody have at least a DD melons.
In the game go Me->Encounters->Restrictions/Goal Weights you can adjust how the npcs behave during the scenes.