Sure, but it's not easy to do as you have to reference elements of the previous story to have it all gel as a sequel as opposed to an chaptered/on-going story.

Right now I'm in the middle of an on-going story that started as a PicFic, everyone wanted the fic to continue, so I did it. As I said, not easy as this recent prompt took the story far from what I originally had going on. The prompts can really make you change direction in the story but it still has to be cohesive and make sense.