In src/map/skill.cpp, find:
case SM_MAGNUM:
...
sc_start4(src,src,SC_WATK_ELEMENT,100,3,20,0,0,skill_get_time2(skill_id, skill_lv));
Add:
sc_start(src, src, SC_FIREWEAPON, 100, skill_lv, skill_get_time2(skill_id, skill_lv));
So it looks like:
case SM_MAGNUM:
...
sc_start4(src,src,SC_WATK_ELEMENT,100,3,20,0,0,skill_get_time2(skill_id, skill_lv));
sc_start(src, src, SC_FIREWEAPON, 100, skill_lv, skill_get_time2(skill_id, skill_lv));
Re-compile server and try it!