Sessions in ASP .NET Core 1.0 and ASP .NET MVC 4

I am doing a series of tests in ASP.NET MVC Core 1.0 to see if it is convenient to migrate the project from MVC 4 to MVC Core 1.0 many of the errors that came out are already fixed.

I have problems passing the parameters that contain sesionManange :

sesionManage.cuenta = WAutofactura_Respuesta.email;
sesionManage.devKey = WAutofactura_Respuesta.DevKey;
sesionManage.schema = WAutofactura_Respuesta.Schema;
sesionManage.mobileKey = WAutofactura_Respuesta.MobileKey;

which the problem is this line:

HttpContext.Session["sessionManage"] = sesionManage;

So my question is How can I pass the parameters of the variable sesionManange to a variable HttpContext.Session in ASP.NET MVC Core 1.0?

EDIT 1
Investigating a bit more in the session variables I found this method to serialize my model of sesionManange :

public static class SessionExtensions
{
    public static T GetObject<T>(this ISession session, string key)
        where T : class
    {
        var json = session.GetString(key);
        if (json == null)
        {
            return null;
        }
        return JsonConvert.DeserializeObject<T>(json);
    }
    public static void SetObject(this ISession session, string key, object value)
    {
        var json = JsonConvert.SerializeObject(value);
        session.SetString(key, json);
    }
}

Which marks me errors in GetObject<T> and SetObject , this is the error that marks me:

  

Extension methods must be defined in a top level static class; SessionExtensions is a nested class