is it possible to extend the Palette with a scroll-bar and to Group Elements by a sub-menu?
I have several different Kinds of Tasks and dataObjectReferences added as shown in the Custom-Elements example and want them to Group, so that These items will be shown by clicking on a sub-menu button (e.g. a small arrow), by Default the items should be hidden. Is this possible at all?
Could you give further Information of how to add a scroll bar, for instance?
That’s outside of the scope of our toolkit. There’s enough information about scrollbars out there. 
If the stock palette implementation is to far from what you desire (functionality wise) you may replace it entirely, too. The stock one clearly shows you how the interaction between palette and the editor is supposed to work.