You can use REPRO from inside REXX to retrieve a record of a KSDS by key, perform whatever task within the REXX, and then once more invoke IDCAMS to replace the record into the KSDS.
You mean to say to write the command:
ADDRESS TSO ,
"REPRO INFILE("VSAM_file") OUTFILE("Seq_file") COUNT(1)"
And then open a seq_file?
But how to edit the vsam record and save on the same VSAM file?