Use oauth2 for authentication and Identity for authorization - asp.net-core

Question

I currently have an aspnet core 2.1 application where authentication and authorization is handled with Identity, where users, roles and claims are all saved in de database.
services.AddIdentity<ApplicationUser, IdentityRole>()
.AddEntityFrameworkStores<ApplicationDbContext>()
.AddDefaultTokenProviders();
Is there a way to switch (only) the authentication to oauth2 and then search the user in the database (when found based on email or login) and then tell Identity that this user is logged in? That way I should not change the authorization functionality in the controllers.
I already tested following oauth2 authentication (provider is github) with success.
services.AddAuthentication(options =>
{
options.DefaultAuthenticateScheme = CookieAuthenticationDefaults.AuthenticationScheme;
options.DefaultSignInScheme = CookieAuthenticationDefaults.AuthenticationScheme;
options.DefaultChallengeScheme = "GitHub";
})
.AddCookie()
.AddOAuth("GitHub", options =>
{
options.ClientId = "my client id";
options.ClientSecret = "my client secret";
options.CallbackPath = new PathString("/Home/Index");
options.AuthorizationEndpoint = "https://github.com/login/oauth/authorize";
options.TokenEndpoint = "https://github.com/login/oauth/access_token";
options.UserInformationEndpoint = "https://api.github.com/user";
options.Events = new OAuthEvents
{
OnCreatingTicket = async context =>
{
var request = new HttpRequestMessage(HttpMethod.Get, context.Options.UserInformationEndpoint);
request.Headers.Accept.Add(new MediaTypeWithQualityHeaderValue("application/json"));
request.Headers.Authorization = new AuthenticationHeaderValue("Bearer", context.AccessToken);
var response = await context.Backchannel.SendAsync(request, HttpCompletionOption.ResponseHeadersRead, context.HttpContext.RequestAborted);
response.EnsureSuccessStatusCode();
var user = JObject.Parse(await response.Content.ReadAsStringAsync());
context.RunClaimActions(user);
}
};
});
Problem is that code like if (_signInManager.IsSignedIn(User)) won't work anymore as this authentication is via oath2.