Connect Unauthenticated Federated Identity ID with User Pool

I'm trying to implement an option in Mobile app (Xamarin) for users to be able to login into App as a guest user.

Right now I'm able to obtain the Federated Identity ID for a guest user. It has a form of AccessKey/SecretKey/Token.

I'm struggling with translating that credentials to accesToken and refreshToken that I get from users, who are generated via user pool.

What could be your suggestions?