To use Japanese input on the Nokia N93, you need to be able to 相撲外:Displaying Japanese on N93.
After you have Japanese display enabled according to the above step, do the following:
You can now run the PiClip application, it allows you to enter the Japanese text in a multi-tap fashion. After you have typed the text, select Options - OK. You can than copy the text to the clipboard by keeping the pen key depressed, selecting the text with the cursor and selecting copy. Then you switch to the app where you want to enter the Japanese text and paste it from the clipboard by keeping the pen key depressed and selecting Paste. It's not super-comfortable but it works.