How to add custom method to auto-generated class in Entity Framework?

I have a class with 2 methods as follow:

public class WorkManagement
{
    public string DoYourWork(Manager manager)
    {
        //
    }

    public string DoYourWork(Employee employee)
    {
        //
    }
}

Manager and Employee are classes generated from database (in Entity Framework). I think it's ugly, for instance, when I need to extend more class, so I want to refactor this into:

 public interface IDoWork
 {
    string DoSomeWork();
 }

public class Manager:IDoWork
{
    public string DoSomeWork()
    {
        //
    }
}

public class Employee:IDoWork
{
    public string DoSomeWork()
    {
        //
    }
}

But how I can deal with auto-generated classes? How I add these thing?

Thank you.