If so I'd suggest a more general 'vulnerability'. IE not just taking excess damage. It could be some weapons are inherently more accurate vs some targets, some weapons cause special effects vs some targets etc. Basically I'd leave it as some kind of open trigger effect that someone could mod later rather than as a fixed damage modifier.
IE 'if target 'A' is hit with damage type 'B' then run trigger 'C'
Trigger C can be pretty much any effect the game can normally generate including instant death, secondary explosions, vision effects etc. Not sure how practical that would be, or if it would be possible to leave any kind of linking system there for someone to add this sort of system in a later mod