It was announced on the official site, as well as at the end of the final TV episode, that Volume 7 of Sekai Seifuku: Bouryaku no Zvezda
, which will be released on October 15, 2014, will include the unaired episode 13. The episode's title is "Shin Zvezda Daisakusen (New Big Zvezda Operation)."
Source: Official site
Sekai Seifuku: Bouryaku no Zvezda - Shin Zvezda Daisakusen