whatsapp

You’d need some third party utility, none of which are endorsed by WhatsApp or Apple, so would be at your own risk.

It is down to the developer to add the functionality, and the functionality isn’t there. How to contact an app developer – Apple Support (UK).

Well there’s no incentive for a 3rd party to provide a service for free. As I said, it’s down to the developer. It’s very simple to transfer WhatsApp chats from iPhone to iPhone, or Android to Android, but not inbetween.