New steel could be added to the dropout, however, the concern would be to keep the drop out cool enough to prevent melting the braze. A Tig welder might be best here, however, I don't have experience with doing this type of repair. We have Forum members that do have experience that hopefully will give you some advice. You could also wrap the brazed area with a wet rag to keep that areas temperatures down. Then file it so the slot is smoother and straight again.