Hi Anton,
The Drag & Drop functionalities in the SDK are effectively related to the cockpit... And are specific to widgets: absolutely not useful for anything else.
Except if SAP modifies the SDK sooner or later regarding this, the only way to go is to "dive" into the Windows SDK (get handler of the dragged object, get handler of the drop to object, convert into B1's SDK object and apply).
Regards,
Eric