I doubt that Apple would let you do this as you are then circumventing the IAP program. You could possibly redirect the user out of the app like Spotify does, but this could still get the app rejected - a small part of the review process depends on which reviewer you get!
Reference in the app store guidelines:
11.2 (source)
Apps utilizing a system other than the In-App Purchase API (IAP) to purchase content, functionality, or services in an App will be rejected