While what you want to do is possible (consider that SDSF reads the contents of the SPOOL datasets), there are no native interfaces that permit this from a REXX procedure. If you must, you can invoke SDSF from the batch and enter commands to it, so that you could add a step ahead of your REXX step to SNAP the SPOOL dataset to a temporary dataset to be read by your REXX procedure. Or, you could imbed the call to SDSF in the REXX procedure (I prefer the separate step). See chapter 11 “Using SDSF in Batch” in the SDSF manual “z/OS V1R4.0 SDSF Operation and Customization” for a description of the interface. You will probably need RACF authority to be able to invoke SDSF and to issue selective commands to it.
If it were up to me, I would just have the step that creates the output write it to a temporary data set (I can’t imagine why it HAS to be written to the SPOOL by the step that creates it) and then just have my REXX procedure read and filter it while writing it to a SYSOUT file created in the REXX step. But that’s just me following the K.I.S.S. principle.