You can specify symbols in MSTJCLxx. Keep in mind that the system does not process symbols in MSTJCLxx in the same way that it processes symbols in parmlib members. Because MSTJCLxx contains JCL, the system processes symbols in MSTJCLxx during JCL processing. The results of symbolic substitution reflect the substitution rules that are in effect during JCL processing.
For details about using system symbols and JCL symbols in JCL, see z/OS MVS JCL Reference.