Actually, I don't think I have the solution for you, but if you wish,
you could try this and tell us what you get...
Execute these commands:
export XMODIFIERS=@im=kinput2 (useless here, I suppose)
kinput2 -xim -kinput -canna & (useless here, I suppose)
and then start kmail or thunderbird and try again...
At least it shouldn't do any harm...
In fact I'm wondering if kmail or thunderbird support importing an address book containing japanese characters.
Maybe you should ask the kmail and the thunderbird teams and see what they say about it. Maybe they have mailing-lists for japanese speaking users?
Good luck anyway!