Rename the EPUB file to something like archive.zip then extract it. Search for the included CSS file (should be named style.css or stylesheet.css). opten the CSS file and paste the contents of
this file in it.
Next, in the same directory, create a "fonts" folder. Drop some Japanese-cabable fonts in it and change the names to match those in the CSS file (or edit the CSS file accordingly).
Now archive everything back in a ZIP file then rename the file something.epub.
It should work this way. I did this with an EPUB containing Romanian special characters and worked for me on firmware 1.5.