Duende.BFF with Strava OAuth: getting data from json body after exchanging token for access and refresh token

Question

I'm developing a Hosted Blazor WASM application that uses Strava's OAuth for authentication. Because I don't want to store the access and refresh tokens in the browser I am using Duende.BFF.

My program.cs is configured like this:

{
    options.DefaultScheme = "cookie";
    options.DefaultChallengeScheme = "oidc";
    options.DefaultSignOutScheme = "oidc";
})
    .AddCookie("cookie", options =>
    {
        options.Cookie.Name = "__Host-blazor";
        options.Cookie.SameSite = SameSiteMode.Strict;
    })
    .AddOAuth("oidc", options =>
    {
        options.ClientId = "clientid";
        options.ClientSecret = "clientsecret";
        options.AuthorizationEndpoint = "https://www.strava.com/oauth/authorize";
        options.TokenEndpoint = "https://www.strava.com/api/v3/oauth/token";
        options.CallbackPath = "/bff/callback";
        options.Scope.Add("activity:read_all,read_all");
        options.SaveTokens = true;
    });

This works perfectly fine but Strava does something after exchanging the token for the access and refresh token. The response is like this:

  "token_type": "Bearer",
  "expires_at": 1568775134,
  "expires_in": 21600,
  "refresh_token": "e5n567567...",
  "access_token": "a4b945687g...",
  "athlete": {
    #{summary athlete representation}
  }
}

I'd like to add the athlete property to the user claims, but I don't see a way to do this currently.

I did try extending these endpoints:

services.AddTransient<ILoginService, DefaultLoginService>();
services.AddTransient<IUserService, DefaultUserService>();

But I think this is already too late in the chain, because the original response isn't accessible here anymore.

Is there a way to extend this somewhere? I tried looking in the Duende.BFF source code but I couldn't figure out where the access and refresh tokens are set.