There is nothing wrong with using frozen fruits and vegetables in a juicer. If the concern is about damaging the motor, I don't think there should be any unless it's a real cheap one, I guess. The best thing would be to check the owner's manual of your juicer to see what it says.
I've used frozen fruits and veggies and even ice cubes in a cheap blender to make smoothies for years now and it still works just fine. By the way, a carrot is harder than frozen fruit