The JOINKEYS subtasks DO NOT have access to the JPn parameters. Only the main task has access. The main tasks and subtasks DO have access to the SYMNAMES data set.
I see it has already been suggested that you set up a SYMNAMES data set with the JPn symbol as a symbol statement and pass that SYMNAMES data set to the JOINKEYS step. You could use a DFSORT/ICETOOL job something like this:
// SET VOL1=SCRTC1
//SUSYM EXEC PGM=SORT,PARM='JP6"&VOL1"'
//SYSOUT DD SYSOUT=*
//SORTIN DD *
//SORTOUT DD DSN=&&S1,UNIT=SYSDA,SPACE=(TRK,(1,1)),DISP=(,PASS)
//SYSIN DD *
//S1 EXEC PGM=ICETOOL
//SYMNAMES DD DSN=&&S1,DISP=(OLD,PASS)
//TOOLMSG DD SYSOUT=*
//DFSMSG DD SYSOUT=*
//IN1 DD DSN=... input file1
//IN2 DD DSN=... input file2
//OUT DD DSN=... output file
//TOOLIN DD *
COPY JKFROM TO(OUT) USING(CTL1)
//CTL1CNTL DD *
//JNF1CNTL DD *
I would NOT recommend using SYMNOUT for SYMNAMES since it has the wrong attributes (FBA/121 instead of FB/80). But if you do use the SYMNOUT data set, you'll have to copy it and modify it to have the correct attributes as well as the correct symbols.