Generally live rock or dry rock for a SW tank should not be used in a FW tank. Apart from the issues mentioned by others, these rocks are usually some form of calcium carbonate based rocks. In FW they will tend to release this into the tank making the water very hard. This might be ok for certain species of fish and plants, but most will do better in softer water.
As usual, the big question comes down to, what do you plan to keep in the tank?