Turn leftover icing into buttercream icing? I made up some basic icing for biscuits: icing sugar + water. 

I have a lot leftover and was wondering whether there was a way to turn it into a buttercream icing? I wondered whether by adding more icing sugar to thicken it, and then adding butter that would work? 

I'm hoping someone else has tried it and knows!

Thanks!
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.