Short answer: 100% impossible. The Wifi Calling app itself has been open-sourced by T-Mobile, but how to completely implement it into the device framework, as well as the SIM authentication method (I believe) is still a mystery. Well, we know HOW it's done, but not how to implement it. I'm not a developer, just stating what I've read.
On older devices, when T-Mobile used UMA instead of SIP (two different protocols) for Wifi Calling, it was possible to do. But as of now, there are huge bounties to try and implement it in AOSP ROMs because no one has been able to do it.