3D object synchronization - element id

I have been trying to use the synchronize from option in the external data tab to update my 3d objects. most of my task assignments were lost after synchronizing and I have found out the reason why:

When I click on a 3d object in my 3d window the yellow object gets selected. I have been assigning my resource tree like this. When synchronizing I use the element id which is shown in the right column. Is there a way to convert all the resource assignments I have made from the lowest level in the 3D tree (so the yellow line) to the parent (one line above) to get the element ID in my user field? If this works the synchronization will be completed without losing the assignments (I checked this for a small sample). 


Kind regards,