Okay guys,
after days of trying different stuff I'll post my own solution in case somebody will find this topic with the same requirement:
The "problem" is that BPMN2.0 works with hidden subtoolboxes, so to make this work you will have to do some "adjustments".
In you UML-profile:1.) you will need to extend your own event-sterotype (MyStartEvent) from
<<metaclass>> Event (the standard UML::Event).
2.) Your extended stereotype will need to generalize the
BPMN2.0::StartEvent sterotype.
In the Toolbox profile:3.) You will have to add your own sterotype in your toolbox-submenu with a certain notation:
NameOfYourProfil::YourEventSterotypename(UML::Event) e.g. Prozessmodel::MyStartEvent(UML::Event)
This will make your item appear in the toolbox and be drag&drop-able into your custom diagram.
Well there is an issue with the icon of your custom StartEvent. It will display the icon of the standard uml event not of BPMN2.0::StartEvent. However, I wasn't able to get this done without overwriting the tagged value of the event:
In your uml-profile:4. Add an attribute to your MyStartEvent stereotype called "icon". As standardvalue paste a path to a 16x16 bitmap image. (I think you'll be able to create a one looking like the BPMN2.0::StartEvent in no time).
The EA help tells that you should take the color "lightgrey" as "transparent" color (rgb: 192,192,192).
That's it

Not that hard, but without the information it's quite much trying.
Good luck,
Regards,
Eric