NO!!!
Don't use fiberglass, its not compatable. What I do is go to the yard and get another bumper, cut filler strips that fit, and then I use a "plastic welding" kit toattatch them. It is basically a super glue, with a sprayon accelerator. Then a company called SEM makes urethane repair material, use this to smoth out your work. Then just prime, blocksand, prime, blocksand and paint.
Ive tried using glass on a caprice bumper before, and it fell right out!!!!