I recommend you not to cover frozen cakes because the fondant will get so humid and sticky that you will ruin de decoration.
Any product you take out of the freezer loses a lot of water, well the same happens with the cakes, the ganache will be hard but a lot of water will be on the outside of the cake, and you need to dry it very well before you cover it, you can use paper napkins.
if you are going to use frozen cakes i recommend you to defrost them at least 4 hours at room temperature before covering. But of course it depends on the size of the cake, can be more time.
Hope it helps Ü (Sorry for my english)
