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I have not looked at these files but I guess it is similar to EU3 pdxa files and your 3D programme wont be able to open them without some sort of script.
Blender can open the cityview 3D files but it cant open the mapitem 3D files.
However an object made from scratch in a 3D program when exported as an .x file can be used as a mapitem for Rome. You can even take some of the .x files from EU3 and use these as a basis from a new map item in Rome.
I found on a few tests that the objects from EU3 have to be scaled up so that you can see them in Rome.