You are looking in the wrong thread. Neither the portrait packs nor the vanilla game have anything to support auto-assigning portraits to existing slaves.
However, the Improved Random Portraits mod has exactly that:
"Added a "Random Portrait" button to the slave customization panel. Clicking this let's you re-roll a portrait for a slave."
If the button isn't present, then you have probably applied the mod wrong, likely the "randomportraits" folder is misplaced.