According to the official website, the seventh BD/DVD volume of Kyoukai no Kanata
will be released on July 2, 2014 and will include Episode 0, titled "Shinonome." Episode 0 happens two years prior to the TV series when the Nase siblings meet Akito for the first time in a mission for Hiroomi. Mitsuki follows her brother on this mission, and their goal is to subjugate Akito.
Other than Episode 0, two more episodes of Kyoukai no Kanata: Idol Saiban! Mayoi Nagara mo Kimi wo Sabaku Tami
will also be included, making it five episodes total.
Source: Official site
Kyoukai no Kanata Episode 0: Shinonome