For converting to F/T rats, you can ask for some dirty mouse aspen from your local pet store... (aspen that their mice lived and pooped in). After the F/T rat is completely thawed, heat it up, and roll it around thoroughly in the dirty aspen to scent it with mouse. And try offering that. This has a high chance of success imo.
After a few weeks of this, your snakes should eventually be able to take the F/T rat without being scented.