I would not recommend the aftermarket approach. I tried something similar with my S5, which actually had wireless charging as an option which was added by changing the back cover. Instead of installing the OEM Samsung charging cover (not even sure if it was still available at that time, but in any event it would make the phone too thick to use a case), I added an aftermarket charging coil which fit inside the existing cover and connected to the same terminals used by the Samsung cover. It worked, but ran hot and over time trashed a couple of batteries so I gave up on it. I suspect an add-on like this on a current phone might have similar results, and it would be worse since the batteries are not user-replaceable any more.
If you really want a phone with wireless charging, buy on that has it built in.