Just open the pepakura software, import the obj file, and it will convert it for you. Then you just have to tweak it and make it the best format possible
I am going to do a roto cast soon, but I would say go for it. If you mess up, you can always try again. I wouldn't go straight into casting a 3d print just because there are bound to be imperfections. Just fill it in and sand and make it perfect, then do it. I beleive in you!