Can you substitute sweetener for caster sugar?

If baking: measure for measure, no (i.e., 1 T caster sugar, 1 T sweetener). Better approach is to find recipes that provide sweetening in other forms (honey, Apple sauce or pureed fruit are frequently-used substitutions for sugar or sweetener). If you are working with a recipe you already have & want to recreate without sugar, try experimenting with the substitutions. Some cooks have devised ways of using sweetener in baked goods, but many believe the finished product has a "metallic" taste.