Adding a new SPOOL volume
First, it may not be possible, depending on the JES2 system parameters and your current configuration. To determine this, examine the output from a $dspooldef command. Check SPOOLNUM= and TGSPACE=. If SPOOLNUM is equal to the number of volumes in the configuration, you can't add volumes. Similarly, if DEFINED is close to MAX in TGSPACE=, you can't add more space.
If it's possible, prepare a new SPOOL volume. The volume serial must be the volume serial prefix displayed in the VOLUME= xxxx output in $DSPOOLDEF, and the data set name must be the data set name in DSNAME= in the $DSPOOLDEF output. SPOOL volumes are not SMS defined, and do not have to be indexed since the VTOC is used only to allocate the SPOOL data set and when JES2 starts the volume. Obviously you must have RACF ALTER to the new SPOOL data set. JES2 will format the data set when it acquires it, so all you have to do is allocate the space. Technically the $sspool command can allocate the SPOOL data set, but it's easier to keep things simple.
Finally, once everything is ready, use the $SSPOOL command to start the new SPOOL volume.