+2 to attack are allowed or at least Our World uses them in several cases, another example of a +2 stunt is
Swing for the Fences (Weapons): +2 to an
attack made with the Weapons skill, but
all his Defense or Block actions made with
Weapons, Fists, or Athletics suffer a –2
penalty until his next turn.
Though I suppose if you have a stunt that extends the dodge trapping to another skill this also could be quite useful.