That's the point, changing the existing fields takes more effort. Try creating a new string attribute for cnt with WSP. Then create a mod file under $NX_ROOT/site/mods/majic/ where you define the logic for the derived attribute. I'm not sure whether you need to use a MODIFY statement or if a OBJECT cnt ... block will be enough as I can't test this for you.
And remember, all the changes you make require you to recycle the SD.