Hi Susan,
it is not possible to purchase an Android app and then want to use the same app on the iOS platform. This is due to the fact that Apple/Google are competitors that do not share information (nor revenues).
If you like to use all of our apps (both on Android or on iOS) as if they were paid then you could consider to buy a subscription with us. During the subscription time you can use any app in its full version. You just will need to include your account information into the settings of the corresponding free app.