Open x0_s0_missstorm1.nss (IGLS) and x0_s0_missstorm2.nss (IGMS)
you will find there a line:
DoMissileStorm(1, 10, SPELL_ISAACS_LESSER_MISSILE_STORM);
remove last ); from it and change for:
,VFX_IMP_MIRV,VFX_IMP_MAGBLUE,DAMAGE_TYPE_ACID);
possible change the ACID into whatever element you want