Estoy tratando de crear una entidad a partir de 2 tablas que no están relacionadas por clave principal

Mesas

CREATE TABLE [employees](
    [ssn] [nvarchar](9) NULL,
    [active] [bit] NULL,    
    [employee_id] [int] IDENTITY(1,1) NOT NULL 
)


CREATE TABLE [sam_employees](
    [ssn] [nvarchar](9) NULL,
    [first_name] [nvarchar](50) NULL,
    [last_name] [nvarchar](50) NULL,
    [skill] [nvarchar](50) NULL,
    ....
)

SQL para generar:

SELECT this_.employee_id  as employee1_0_0_,
       this_.ssn          as ssn0_0_,
       this_.active       as active0_0_,
       this_1_.first_name as first2_1_0_,
       this_1_.last_name  as last3_1_0_,
       this_1_.skill      as skill1_0_
FROM   employees this_
       inner join sam_employees this_1_
         on this_.ssn = this_1_.ssn
WHERE  this_.active = 1

Mi mapeo actual:

public class EmployeeMap : ClassMap<Employee>
{
    public EmployeeMap()
    {
        Table("employees");
        Id(x => x.Id, "employee_id");
        Map(x => x.SSN, "ssn");
        Map(x => x.IsActive, "active");

        Join("sam_employees", mm =>
                                  {                                          
                                      mm.KeyColumn("ssn");
                                      mm.Map(xx => xx.FirstName, "first_name");
                                      mm.Map(xx => xx.LastName, "last_name");
                                      mm.Map(xx => xx.Skill, "skill");
                                  });            
    }
}

Pero con este mapeo tengo esa condición de unión en this_.employee_id = this_1_.ssn

Sé que esta pregunta se hizo antes, pero no encontré una buena respuesta, solo una solución alternativa para usar Ver en el lado de la base de datos en lugar de tablas.