I don't think we can say for sure, but I'm hopeful. Google has typically pushed the same software to each of their Nexus devices that receive the update. I had read somewhere than the Nexus 5 has all the hardware it needs for both VoLTE and IMS (Wifi Calling), so I imagine a software update could do it just like Apple did with the iPhone.
Google did make the Google Now Launcher exclusive to the Nexus 5 for some time though, so I don't know for sure if we would be getting WiFi calling at the same time as the Nexus 6 or if we may get it later.