public class ViewModel
{
    public string Username { get; set; }
    public string Password { get; set; }

    public User ToUser(User user)
    {
        user.Username = this.Username;
        user.Password = this.Password;
    }
}


========================
Then in action method
========================

public ActionResult CreateUser(ViewModel viewModel)
{
    User newUser = new User();
    newUser = viewModel.ToUser(newUser);
}

public ActionResult EditUser(ViewModel viewModel)
{
    User existingUser = userRepo.GetUser(viewModel.Username);
    existingUser = userRepo.ToUser(existingUser);
}