public interface IConcreteEntityA : IActivatableEntity
{
    //...
}

public interface IConcreteEntityB : IActivatableEntity, IAuditEntity //not possible!!!
{
    //...
}

public interface IAuditEntity
{
    DateTime CreatedOn { get; set; }
    DateTime ModifiedOn { get; set; }
}

public interface IActivatableEntity
{
    bool IsActive { get; set; }

    void Active();
    void DeActivate();
}