[TYPO3] change/add content wizard items

[Patric Pesch]

Hi list,
i'd like to change the content wizard items
(the "New content element" wizard). I want to add
the content-element "header" and to change the element
"Text with image below" to "Text with image to the left".

How can i do this? With
TCEFORM.tt_content.CType.removeItems = ...
i can remove any item, but how can i add some?

Thanks for any hint!
Patric