How can I edit the backend of firebase cloud function extension?

Question

I am coding a project similar to patreon.com and my goal is to let multiple members create their own subscription plans and sell it.

I came across a firebase extension for stripe payments - https://firebase.google.com/products/extensions/stripe-firestore-stripe-payments

The problem with this extension is that I can only create a 1 premium membership that adds custom claims to the auth object and I can validate it like this:

export default async function isUserPremium(): Promise<boolean> {
  await auth.currentUser?.getIdToken(true);
  const decodedToken = await auth.currentUser?.getIdTokenResult();

  return decodedToken?.claims?.stripeRole ? true : false;
}

That means that even if I have 100 different subscriptions, I can only attach single boolean value, which is useless.

I went to the source code and I found this snippet:

  // Update their custom claims
  if (role) {
    try {
      // Get existing claims for the user
      const { customClaims } = await admin.auth().getUser(uid);
      // Set new role in custom claims as long as the subs status allows
      if (['trialing', 'active'].includes(subscription.status)) {
        logs.userCustomClaimSet(uid, 'stripeRole', role);
        await admin
          .auth()
          .setCustomUserClaims(uid, { ...customClaims, stripeRole: role });
      } else {
        logs.userCustomClaimSet(uid, 'stripeRole', 'null');
        await admin
          .auth()
          .setCustomUserClaims(uid, { ...customClaims, stripeRole: null });
      }
    } catch (error) {
      // User has been deleted, simply return.
      return;
    }
  }

I don't fully understand this code, but I think this is where the boolean value is assigned.

Would it be possible to somehow edit this source code, so that instead of boolean value, I could store subscription plan ids in Array , so that in the front end I could validate and allow customer to access users content only if he has active plan in that array ?