Aperture 2 vs Custom EXIF field

Hello,

I have a lot of pictures with custom 'rating' EXIF field. I tried to import this pictures to Aperture.

The import was successfully, but have a litle problem. I see the custom field in metadata others and general part, but:
- this field is readonly (this is a smaller problem, i want not use later)
- the Aperture didnt read up this field value, when i check with other EXIF SW (exa. exiftool) i see the correct value...

How can i import this field?

Thanks your help...