How do you add a buff on all units in area?

I'm not sure how to.

Buffs can only be given to units by casting spells on them that offer the desired buff.

yeah.... maybe just use a dummy spellcaster. with the desired spell having a cooldown of 0.. then just like pick up everyunit in X area and order dummyunit to cast Xspell on picked unit.... or something like that

Get Vex's spell kit... very easy example in there.

I want to add stun like bash for all units in xrange of targeted point of ability being cast.

Do you mean the caster system?