You can also use a method or the IClonable Interface to clone your data.
I prefere my own method with a clear name (shallow or deep clone):
public sealed class Employer
{
    public int IDNumber { get; set; }
    public DateTime RegDate { get; set; }
    public List<BioData> BioInfo { get; set; }
    public Employer DeepClone()
    {
        Employer loClone = new Employer()
        {
            IDNumber = this.IDNumber,
            RegDate = this.RegDate
        };
        if (this.BioInfo != null)
            loClone.BioInfo = this.BioInfo.Select(item => item.DeepClone()).ToList();
        return loClone;
    }
}
public sealed class BioData 
{
    public int IndexID { get; set; }
    public string Description { get; set; }
    public int Age { get; set; }
    public double Salary { get; set; }
    public BioData DeepClone()
    {
        //Can also use here
        //return this.MemberwiseClone() as BioData;
        return new BioData()
        {
            IndexID = this.IndexID,
            Description = String.Copy(this.Description),
            Age = this.Age,
            Salary = this.Salary
        };
    }
}
UPDATE
To copy the entries from an existing list in the same list, you can use LINQ.
(ToList is necessary):
List<Employer> loList = new List<Employer>();
loList.ToList().ForEach(item => loList.Add(item.DeepClone()));