If you are running ICKDSF V17. Of course you need free space after the vtoc/index to expand it.
The following is an example of expanding the VTOC and the Index using the EXTVTOC and EXTINDEX parameters.
//EXAMPLE JOB
// EXEC PGM=ICKDSF
//VOLDD DD DISP=SHR,UNIT=3380,VOL=SER=TMP121
//SYSPRINT DD SYSOUT=A
//SYSIN DD *
REFORMAT DDNAME(VOLDD) VERIFY(TMP121) EXTVTOC(200) EXTINDEX(16)
/*