You can just dip in FW, but it can be pretty rough on the polyps.
It's better to buffer up the FW PH until it matches the PH of the water the zoas are coming out of, as well as matching temperature.
There are alot of dip products that can be used with SW as well, ie Lugols, Reef Dip, etc that have worked well for me, and are much gentler than FW dips.