I think you are correct that this cannot be done; these values would be hardcoded into the report at design time. Instead you will likely need to use a SQL connect rule and pass these values in as params.
If you let me know exactly what you are trying to accomplish rather than how you are trying to accomplish it perhaps there is a better way altogether to do what your activity is doing.
PS Please include what version of Pega you are working in as this would affect the possible solutions.
The requirement is like this.. there is a pega soap service that have configured for search data from database. The request that pega getting, contains the sorting column and maximum recods needed in response.
What I have tried is, in the service activity after the request data mapping step I have tried to pass parameters to pxRetrieveReportMetaData activity and after that I'm calling pxRetrieveReportData..
The version I'm using is 7.1.7