I made butter cream icing for a cake and I had a ton left over and I have to do cookies can I make this icing into royal icing by just adding meguine powder? Hope someone out there can help..have a nice weekend audrey

No, I'm sorry. They are two totally different things. The biggest difference is royal icing has no fat in it (butter or shortening). Traditional royal is just egg whites and powdered sugar, but some recipes replace the whites with meringue powder and water, among other things.