Once more let us clarify exactly what JCL is.
it is J
By itself it does nothing. It is used to invoke the chosen program and to define which datasets are to be used by the DD names associated with the chosen program.
JCL does not have magical powers that solve all problems, JCL is merely the vehicule for telling the processor which program you
want it to execute./RANT OFF
So, by using MORE
than just JCL you can use the IDCAMS REPRO command to copy the first 589 records, and then with a second set of statements copy from record 591 until the end of file.
Also, you may wish to read the sort product documentation, because I believe that you are incorrect in saying that you need to know anything about the record except its relative position within the file. I know that this can be done using DFSORT, but for SYNCSORT I do not know as I have not used it in many many years.
to read the DFSORT smart tricks publication.