I am not sure how to create a setting for this requirement but a very crude way to do this for a few dimensions would be to use 'Fake Dimension' and assigning the required number of spaces before the actual real value.
I did have a go at setting the dimensions 'prefix' as 'R ' (or similar with diameter symbol) but Catia seems to ignore the spaces in this case.
Do let us know if you find a way to 'set' this although I fail to see when I might feel the need to use it..!
Rishi Mandyam
Automotive Design Engineer
Catia V5 R12 - latest
Chelmsford, UK