you will see that S(9) LEADING SEPARATE is equivalent to DFSORT's FS format. You can't use that directly in SUM. You can convert the FS value to ZD, SUM it and convert it back to FS. However, it might be easier to use OUTFIL SECTIONS/TRAILER3/TOT with FS instead.
If you show me an example of your input records and what you want for output, I can show you how to do what you want.