Can frozen food be returned to the grocery store if it is still frozen?

You can try. However, nearly every retailer will not accept chilled or frozen goods back unless there is product issue with them i.e. out of date on day of puchase etc. The reason is that these goods need to be maintained in their environment to keep them edible. Once purchased the retailer cannot know if they have defrosted or gone over their chilled safe temperature. If you return someting because its not what you want or wrongly purchased they most certainly will not replace or refund on them as they cannot resell the item. It will rely on the retailers good faith.