I think i've found the solution myself:
I did not thought that the compositeData in custom controls are so clever:-) if i pass the object as an interface (e.g. com.app.IProzessitem), it works..
This really fits well in my design pattern, as i'm working on a quite big Workflow Application.. All the ProzessItems are different, but all of them have similar functions (select Company, forward/return, etc.).. So like in the java backend, i can also pass the interface in the GUI.. this will make my customcontrols more reusable..love it