[Cougra]

Mollies have an amazing ability to adapt. I have seen people put them from a freshwater tank right into a fully saltwater tank and live, however I think it would be best slowly to raise the salinity over the course of a week or two. If the Mollies have been kept in a system that has a bit of salt added all the time then they can be acclimatized faster then one that has been kept in a completely salt free tank.

Not all freshwater fish can be adapted to saltwater conditions. There are primary and secondary freshwater fish. Primary freshwater fish have evolved in freshwater and as such can not handle high levels of salts in the water for long periods of time.
Secondary freshwater fish are fish that started off in the oceans and slowly moved inland. These fish tend to do better with a bit of salt in the water but still can't really adapt to full salwater conditions.
Then there are brackish water fish that can adapt to full saltwater and full freshwater conditions but still tend to do better somewhere between the two extremes. (This is where mollies fit in.)