Is there a way to enable Qi Wireless charging for Nexus 6P or Nexus 5X? They had wireless charging in all the previous Nexus models and I'm sure it's a hidden feature in there somewhere.
No, out of the box it won't. The reason is that wireless charging requires specific hardware to work. Indeed it is possible thanks to electromagnetic induction; in simple terms: there is a coil inside both charger and phone, connected to the battery. The charger generates an alternating elecromagnetic field in the coil, with which the phone's coil pairs, generating current which charges the battery. It will be possible, although, if some firm produces specific covers with the necessary hardware embedded, which already exist for some flagship phones, so there will be a workaround.
Seems that you can hack the Nexus 5X and integrate a Qi Receiver into the phone: