Depending on the nature of the kanji being used, Chapter 5 of the Japanese Language Kit Input Guide ("Using Dictionaries") may provide a solution. Otherwise see below:
When Kanji TrueType was introduced, User Gaiji (user defined characters) was dropped, because of the complexity of adding them to the outline fonts.
To our knowledge, the only program that produces User Gaiji is a 3rd party package from Enfour that patches the fonts.
For additional details on third party Japanese Language Kit products, contact a Japanese Language Kit reseller in your area, or call SystemSoft.
The Tech Info Library article titled "Locating Vendor Information" can help you search for a particular vendor's address and phone number, such as Enfour or SystemSoft.
Note: This article provides information about a non-Apple product. Apple Computer, Inc. is not responsible for its content. Please contact the vendor for additional information.