public class EntProducts
    {
        public Db.Products.Products Main { get; set; }
        public Db.Products.Categories Category { get; set; }
    }

    class SvcProducts
    {
        IQueryable<EntProducts> Products
        {
            get
            {
                return
                    from p in DbServices.Products.Products
                    select new EntProducts()
                    {
                        Main = p,
                        Category = p.Categories
                    };
            }
        }
    }

    class SvcProducts
    {
        IQueryable<EntProducts> Products
        {
            get
            {
                return
                    from p in DbServices.Products.Products
                    select new EntProducts()
                    {
                        ProductID = p.ProductID,
                        ProductName = p.ProductName,
                        CategoryID = p.CategoryID,
                        CategoryName = p.Categories.CategoryName
                        // И ещё XXX свойств
                    };
            }
        }
    }