Insert user via Identity ASP.NET

Question

I am developing an application in ASP.NET where I am using Identity.

I have some initial functions from a External API that are called when I am validating the Token of this API via controller.

The fact is I need it inside of mine controller, check if the user is already registered on my system and if not, I need to register it, all this within this controller, so that I can redirect the user to certain places.

Follows excerpt from my code

        Rootobject DadosUser = new JavaScriptSerializer().Deserialize<Rootobject>(RetornoUser);

        var IDUsuario = DadosUser.id;
        var UsuarioLogin = DadosUser.nickname;
        var UsuarioPrimeiroNome = DadosUser.first_name;
        var UsuarioSegundoNome = DadosUser.last_name;
        var UsuarioEmail = DadosUser.email;



        ViewBag.UsuarioLogin = UsuarioLogin;
        ViewBag.NomeCompleto = UsuarioPrimeiroNome + " " + UsuarioSegundoNome;


        //INSERINDO NOVO USUARIO


      // Aqui preciso verificar se o usuario recebido ja existe no cadastro, e caso não, preciso cadastra-lo