Can I integrate another design pattern to my existing Builder design pattern?

I have created a simple project using the Builder design pattern but I would like to add one more, maybe integrate the Singleton patterN. I am very new to design patterns, so I'm sorry if this question sounds a bit stupid. I will like to experiment with combining design patterns but I'm not sure what would fit the already existing project.

namespace BuilderProiect
{
    class Program
    {
        static void Main(string[] args)
        {
            var builder = new EmployeeTestBuilder();


            var employee = builder
                 .WithName("John")
                 .HasAge(23)
                 .HasGrossSalaryOf(2800)
                 .Build();

         
            int actual = (int)employee.CalculateNetSalary();
           Console.WriteLine("The name of the employee {0} who was born in {1} with gross salary {2} which is actual salary of {3}\n", employee.Name, employee.BirthDate, employee.GrossSalary, actual);

        }
    }
    public class Employee
    {

        public string Name { get; set; }
        public DateTime BirthDate { get; set; }
        public double GrossSalary { get; set; }

        public double CalculateNetSalary()
        {
            var age = DateTime.Now.Year - BirthDate.Year;

            if (age < 18)
                throw new InvalidOperationException("Age less than 18");
          
            if (age >= 18 && age < 30)
                return GrossSalary * 0.8;
            else
                return GrossSalary * 0.85;
        }
        public class EmployeeTestBuilder
        {
            private Employee _employee;
            public EmployeeTestBuilder()
            {
                _employee = new Employee();
            }

          /*  public EmployeeTestBuilder Default()
            {
                _employee.Name = "Mark";
                _employee.BirthDate = new DateTime();
                _employee.GrossSalary = 1000;
               
                return this;
            }*/

            public EmployeeTestBuilder WithName(string name)
            {
                _employee.Name = name;
                return this;
            }

            public EmployeeTestBuilder HasAge(int age)
            {
                _employee.BirthDate = new DateTime(DateTime.Now.Year - age, DateTime.Now.Month, DateTime.Now.Day);
                return this;
            }

            public EmployeeTestBuilder HasGrossSalaryOf(double salary)
            {
                _employee.GrossSalary = salary;
                return this;
            }

            public Employee Build()
            {
                
            
                return _employee;
            }
        }
    }

}