If you elected not to install the Japanese Input Method when you installed BeOS, and later decide you’d like to give it a try, you don’t have to install the OS again just to get the option. Look in /boot/beos/etc/install/_japanese_install_ on your BeOS CD. Then, taking care not to overwrite existing folders, copy the contents of these folders to corresponding folders on your hard drive. Reboot or see this tip and the Japanese Input Method will be enabled.
Tap Alt+Spacebar to switch into kanji input mode system-wide, or use the new replicant you’ll find in the Deskbar.