They can be fixed, but you will be there for months with a sledge hammer. Better to use a press to straighten them out. I have done this with satisfactory results. They won't be perfect, so mark which one is the straightened one and use it on the back, that way you won't feel it through the steering.
Most wheels are made from 6061 aluminium which can be straightened without cracking if the buckle isn't too bad. If you are unsure, probably worth getting it flourescent penetrant inspected before you put the tyre back on.