You can only used fields that are created or calculated before the report is generated for sorting.
You would eitehr have to make these values be in the main report and be able to be generated using the insert Summary Function and proper groupingĀ and you can sort using TOP N or you would have to calculate them outside of crystal (e.g. a stored proc) before the report is generated and use that as the source and you can sort using the stored proc. fields.