If you have a non-shock type bumper, I guess it's doable but looks like a hassle to me. Why not just get the proper filler? If you have the shock type bumper, I think you're out of luck. How about cutting longer slots in the bracket?
The brackets, where they bolt to the frame, have slotted holes. If you make those slots longer, you would be able to slide the bumper closer to the grille and/or rear trunk panel as the case may be.