(THERE'S PROBABLY MOD THAT DOES THIS)
What do prisoners do? They pace back and forth in their cells, getting a paste meal, changing clothes occasionally and chatting at the table if your nice and getting their organs ripped out if your not. They do this until a slaver comes or they're convinced to join. Why not have them do simple, rigorous tasks no one wants to do! Maybe in the Prisoner tab, they'll be a 'Slavery' option. Slaves will go outside finally and do tasks like: harvest, sow, and cut plants; mine; do (very) simple crafting in a sweatshop you built; hauling; and cleaning. This will have the benefits: Give the prisoners a chance to go outside, improve skills, and do jobs your colonists are too busy to do. Slavery will have the following downsides: Increased chance of prison break, prisoner-colonist rivalry and violence, and colonists won't do those tasks as very often, prisoners might not like their enforced labor and refuse to do it and get mood penalties (gasp). As stated previously, this might've been done before in M O D S.
With that feature I can finally create the massive cotton feilds I want to.
Check out the Prison Labor mod, pretty much adds just what you're looking for. You'll also want to use the creators Lock mod too which will allow you to restrict which doors your prisoners can use.