Modify checkout shipping to add pickup locations in combination with the CarrierService api

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Is it possible to add extra data to shipping rates? I can only send a response with specific data with the CarrierService API.

I could modify the theme, but then I read this in app requirements:

5. Security and merchant risk
  A. Security
    5. Your app must not alter or modify Shopify's checkout.

Does this mean I can't touch the checkout page at all?


The idea here is to show a map, and a select dropdown with various pickup locations, for each shipping rate that has this enabled.
This data also has to somehow get added to the order together with the other shipping information. Or at least a way so I can catch it and save it in the app. Probably with a webhook then? So how would I add that data to the webhook? Or do I have my script send the selected data, listening to when the customer presses place order?