Yep, no problems there. I'd probably soak them in hotwater and oxiclean to remove any remaining detritus, but honestly, other than decaying organics, a good rinse in water will remove any lingering salt, and there isn't much else that would be introduced to a FW tank from SW bio-balls.
Make sure you have a good pre-filter ahead of the sump--this will minimize the solid wastes that get trapped in the bio-balls.