I would be pretty hesitant to use a water softener with the water for your frogs. Some of those softener pellets have added chemicals that aren't safe for aquarium use, so they probably wouldn't be healthy for frogs either. My parents have a water softener system in their house and when I visit my skin breaks out in a terrible rash if I shower or wash my hands in their water! Something in the softener pellets just aggravates my eczema, so I can't imagine what it would do to a frog's sensitive skin.
I agree with Bill that a natural method would be better if you do want to soften your water.