Is this about a wookie wrestling a Rancor? Adding setback dices assumes that the wookie still has a chance to succeed.
I'm with EldritchFire on this. This should simply fall under the rule "Automatic failure/success".
C'mon now, that's not very "yes, but..." of you...
If a player wants to do it after you say how many challenge dice or setback dice or whatever you decide they get, it's their funeral.