How to update ModelState?

This is a controller action that I call with ajax post method:

    [HttpPost]
    public ActionResult Add(Comment comment)
    {
        if (User.Identity.IsAuthenticated)
        {
            comment.Username = User.Identity.Name;
            comment.Email = Membership.GetUser().Email;
        }

        if (ModelState.IsValid)
        {
            this.db.Add(comment);
            return PartialView("Comment", comment);
        }
        else
        {
            //...
        }
    }

If the user is logged in, submit form doesn't have Username and Email fields, so they don't get passed by ajax call. When action gets called ModelStat.IsValid returns false, because these two properties are required. After I set valid values to properties, how do I trigger model validation to update ModelState?