Thanks a lot guys for replying!
I overlooked something!
The AVCHD file structure was just for the CX730. I misunderstood the content since i just found out that some (CX730-) footage was recorded on the SD card that came out of the D3100. In the usual ‘100D3100’-folder I found the .mov files.
Still strange that the ‘open archive’ functionality is gone with an update of FCPX, because I’m still looking for a solution of the (huge batch of) CX730 footage.
Thanks again the advice you’ve already given!