Wow, lot of people with a lot of really awesome ideas. I'm really glad that so many people got on board with this!
Anyways, here's what I've decided to go with: Kallig, being the sick and deviant lord of the Sith that he was (also not a Sith supremist), decided he would use Dark-Side Alchemy (or maybe just old fashioned baby-making juice) to create a hybrid Twi'lek. When he was killed, another Sith (pressumably Tulak Hord) enslaved the child as a kind of final insult to Kallig, and also because he hated impure sith.
Does everyone think that makes sense? I'm not gonna go into specifics about how the baby was made because I'm still not 100% sure if Twi'leks and Humans can mate naturally. If you have any further suggestions please let me know! :D