Seems that the T1 dictionary are msdict (from
http://www.mobisystems.com/ ) dictionaries... so maybe would be easy add new ones...
I guess the first step would be extract Japanese dictionaries from the Japanese T1 and try to put them under European/American T1 version and see if them will work...
Of course both must be rooted before...
The dictionaries has the .msd extension. I'm looking for some file like this, but I still didn't found...
Regards...