...
More sophisticated requirements can be met by implementing native models, views, and controllers in the CakePHP framework. Specifics are beyond the scope of this document, however note that while the Registry knows about and can manipulate simple extended attributes, it will not know about complicated extended attributes except to the extent that you modify the appropriate code.Currently, there are no standards for implementing complicated extended attributes. As such, future versions of the Registry may not be compatible with your implementationThe best way to do that is probably to write a Plugin.
Extended Types
Extended types allow custom types to be defined for a given attribute. Not all attributes support extended types. Defining custom types for supported attributes is done by selecting "Extended Types" from the CO menu.