Turn leftover icing into buttercream icing?
It will if you add more icing sugar but you may end up with a lot of buttercream depending on how much of the icing you have left. A basic ratio is 4 cups icing sugar to 1 cup of butter and 1/4 cup of liquid which makes 3 cups of icing. You may have to add more icing sugar, then more butter to compensate for the water if there's more than 1/4 cup in your original biscuit icing.