FanFic I'm pretty sure has been allowed for as long as I've known about NaNoWriMo, which has been a few years. The only goal I really knew you HAD to do was write 50k in 30 days. I make it a personal goal NOT to do fanfiction (because finishing fanfic stories isn't a problem for me but doing an original story is a HUGE problem for me, so that is the goal I want to overcome.)