It depends on the recipe. In cakes made by the rubbing-in method, you can use the white sugar as it is. If you need a finer sugar, it is possible to grind white sugar in a blender or coffee grinder: the result is more like icing sugar than caster sugar but it would be acceptable for making biscuits, pastry, icings and so on. Home-ground sugar is not, however, suitable for creamed cake mixtures as the volume of sugar will be reduced and result in a cake with a hard crust.