"Negotiations have arrived at their objective," the Nikkan Sports newspaper was told.
Joseph played for Sanix in Japan, can speak Japanese and fits the JRFU criteria about hiring a coach with international experience and an understanding of the rugby culture in Japan.
One of the sticking points would have been that Japan's next tests are against Scotland next June when Joseph is still coaching the Highlanders towards the final stages of the Super rugby round robin games.
However Super rugby takes a break in June and the Highlanders resume against the basket-case Kings in South Africa. Joseph may have been able to negotiate a mid-series release for a few weeks to accommodate his Super rugby contract and Japanese coaching hopes.
Meanwhile Hammett, who beat Joseph for the Hurricanes job in 2011, has been ratified as coach of the Sunwolves. They are unlikely to make the playoffs in their debut year so Hammett will then be able to work as assistant coach of the Tasman Makos in the national provincial series.
Veteran lock Hitoshi Ono has signed on for the Sunwolves after the 37-year-old was part of the RWC squad and wanted another challenge late in his career.
Two other RWC squad members, Yoshikazu Fujita and Kenki Fukuoka have declined offers to be part of the Sunwolves to prepare to be part of Japan's sevens squad for the Rio Olympics.
Two Rebels players who went to the RWC, Shota Horie and Keita Ingagki, have signaled their intentions to play for the Sunwolves along with wing Akihito Yamada, Amanaki Mafi and Andrew Durutalo.