We need to use UmConverter. This software is a standalone converter between ownformat(.bos) and fbx(.fbx). You can get it from http://umconv.render.jp/. I currently use version "DOWNLOAD__ver20150131_7.5MB".
Cache:
"User preferences":
"Install from file":
Choose the "io_scene_box_fbx.zip":
Mark the checkbox to activate it and "Save user settings":
Done!

You can now just drag and drop the .FBX file into a Unity project.
