You can find interesting this rather lenghty discussion.
Eugene Tyson's approach (post 78 from discussion above):
File>Export>PDF> Acrobat 6 or above and Create Layers
Open PDF in Illustrator
File>Export>PSD and select write layers
After all, consider doing it manually: export each layer to separate pdf, then combine them to one Photoshop file.
If you do that, export as PDF/X-4 without resampling and without compressing any image.
Thank you all for your support!
I finally followed winterm suggestions and found this interesting article with more information (german):
Have a good day!