No, this is not possible. Grosh uses the in-app purchase mechanism provided by Google and Apple. You actually buy the premium subscription using an in-app purchase from Google or Apple, we just get a bank transfer once per month with a share of the total sales. Google and Apple do not share information with each other about in-app purchases, so therefore our app cannot validate your purchase even if we knew you had premium. Until this changes, the answer is unfortunately no.
