Can frozen egg whites be used in a macaron recipe?
Yes, you can, but the results may not be the best, when it comes to tenderness and aeration of the finished (baked) product. Coagulated egg products are mainly used for omelettes, scrambled eggs and souffles especially for mass production, or great quantities.

In cooking, but especially in confectionery making, fresh eggs are usually used, especially for preparations that are very delicate, like macarons.