RTW tool can not price First class itinirary.
You may contact the assistance for asking how to book in First , but AFAIK, booking is only possible in the origin country, for your travel in Japan.
However, several years ago, it was possible with south african TA specialized (for travel from SA, of course)
Perhaps the rules have changed as it seems possible to book and pay any travel with the tool (in eco and in biz)
Tell us what they told you by phone or mail.