He creado mi Authenticaton Manager personalizado para Spring Security que se parece a esto

   public class AccountAuthenticationProvider implements  AuthenticationProvider{

    @Autowired
    private AuthenticationService authService;

    @Override
    public Authentication authenticate(Authentication authentication) throws AuthenticationException {

        String userName = authentication.getName();
        String password = (String)authentication.getCredentials();

        if(authService.isValid(userName,password)){
            List<GrantedAuthority> grantedAuthorityList = new ArrayList<GrantedAuthority>();
            grantedAuthorityList.add(new SimpleGrantedAuthority("ROLE_USER"));
            SecurityContext securityContext = new SecurityContextImpl();
            return  new UsernamePasswordAuthenticationToken(userName,password);
        }

        return null;
    }


    public void setAuthService(AuthenticationService authService) {
        this.authService = authService;
    }

    @Override
    public boolean supports(Class<?> authentication) {
        return true;
    }

}

pero, ¿cómo creo mi propio objeto UserDetail personalizado? Lo usaré para almacenar valores relacionados con la cuenta