Hi @dominikjasek ,
Thank you for reaching out!
You can add custom claims to an Access token through Actions with the following function:
api.accessToken.setCustomClaim(name, value);
You can find more details in our following Knowledge Article right here.
Also, this doc could also serve useful in regards to Action triggers in a post-login Action : Actions Triggers: post-login - Event Object
Hope this helped!
Gerald