Kabuto explained that a ninja can't be summon in 2 cases:
1- The death's gos already took the soul(like the kokages);
2- If a edo-ninja (like zabuza) was sealed;
But he learned how to use edo-tensei. He could brake the seals to use they again (third raikage, Hanzo, and all the others that were sealed)
1- The death's gos already took the soul(like the kokages);
2- If a edo-ninja (like zabuza) was sealed;
But he learned how to use edo-tensei. He could brake the seals to use they again (third raikage, Hanzo, and all the others that were sealed)
but he doesnt have their dna, Kabuto has
since he has all these eyeballs, he can extract some DNA from them 
