So, as of now, it can't be done officially. By there's a Google app called Hangouts Dialer that offers free VoIP, and you can set it so that it shows your Verizon phone number in the caller ID.
I think I should mention this weird thing with my 6P...
My carrier doesn't support Wi-Fi calling (it's a local one that is basically T-Mobile's bands) and the setting shows up nowhere, but if I do a search of my system settings for "call" one of the options us Wi-Fi calling. I can enable it...