Finally I hope, there will be a standard. In IDimager there is a possibility to choose the way to read and write the hierarchical structure.
The problem is, that even adobes bridge cs3 can't read the hierarchical structure in a right way. But I've got an answer by phase one support team:
Looking at the file itself it seems as though Media Pro is reading the file as the XMP standard allows, as we can see a similiar conflict when looking at the XMP editor in Photoshop. Regardless I will suggest we look into proper accomodation for this type of Keywording with a future release.