Basically, the various items/spells/skills/races/whatever are all implemented as "props" defined in the character sheet XML. These props can have a wide range of attributes (including arbitrary values associated with them), as well as have associated actions.
As such, it isn't practical to include built in editor support for creating new ones (for example, a class may have a whole list of actions based on each additional level gained in the class, so may end up being a whole page of XML). To get around this, a character sheets can have "extensions" (basically the exact same name with "+ExtensionName" before the .xml) that can be added to the given character sheet at run time. This avoids having to edit an already huge file that provides basic support, and then having to merge in changes, deal with conflicts from other users, etc... Not to mention that trying to edit large amounts of stuff like this on the device is painful to say the least.
So to add in your own custom stuff you'll need to create an extension XML file (which you can then sync to the device using iTunes)