I believe the AddUserToRole method does the job at the database level. Although this is likely to happen as well, what you need to do is update your current identity.
Short answer: translate IPrincipal to ClaimsPrincipal and pass the IIdentity value to ClaimsIdentity. Then you can simply add a request.
ClaimsPrincipal currentPrincipal = (ClaimsPrincipal)this.User;
ClaimsIdentity currentIdentity = (ClaimsIdentity)currentPrincipal.Identity;
currentIdentity.AddClaim(new Claim(ClaimTypes.Role, "Role2"));