No, they cannot. There are some fish which are naturally brackish fish... that is, they were used to a brackish environment before being used as pets. Mollies are a good example of that kind of fish, as are a few other livebearers.
You cannot just take any freshwater fish and try to turn it brackish though. They will suffer...some moreso than others. Like I said, if it were some of the livebearers out there that you were trying to make brackish, they may survive but if it was corydoras you wanted brackish, I think you'd have a mess on your hands.