Do you mean if hypo dosen't work can you f/w dip? I have used a f/w dip in extreme cases of external parasites on 2 occasions to give temporary relief(ph and temp adjusted with water well oxygenated), and then put the fish into hypo. A f/w dip will only treat the fish. Once the fish is introduced back into the tank the parasites still exist.
I have used hypo 3 times with good results and prefer it over copper, which I have used successfully as well but more stressful.