The 6-foot-5 defenceman was one of two first-round picks for Buffalo in the 2013 draft, selected 16th overall.
In three preseason games, Zadorov has a goal and four penalty minutes. Last year in 63 games for the Ontario Hockey League's London Knights, he had 25 points and 54 penalty minutes.
Zadorov will have an opportunity to make the team. If not, he will be returned to the Knights.
"Watching Nikita throughout training camp and the preseason has solidified our opinion that he will be a good, young hockey player for our organization going forward," Sabres general manager Darcy Regier said in a statement. "He has exceptional presence on the ice and we look forward to seeing him continue to develop as a player."