The inset parameter will let you do this... somewhat. For example: an inset of ccp(-100,-100) pad out the containment rect by 100 on all sizes, effectively increasing your world size by 200 horizontally and 200 vertically.
You could also use this alternative api which would make a rect who's origin is at 0,0 but with the width and heigh you give it.
-(void) addWindowContainmentWithFriction:(cpFloat)friction elasticity:(cpFloat)elasticity size:(CGSize)wins inset:(cpVect)inset radius:(cpFloat)radius
default radius is usually one, its just the thickness/2.0 of the line segments