The Lumia 950 currently has the slow motion option for video recording at 120 fps. Would it be possible to get it at 240 fps in a future through a software update? Or simply the camera hardware won't handle it?
Hardware can't handle it? sure it can, and probably will get an update in a future, but it is not so important, as quality, stabilization and other, but sure it will come, like 4k came for L930.
Are you an engineer who knows the specifications to a T?
The hardware may or may not support it.
It IS possible for it to be done with an update like 4K as you said, but we don't know if the hardware is capable for sure because 240 fps is pretty speedy. Worth noting that the iPhone 5s can only do 120 fps while the 6 can crank it up to 240. I can say that's hardware.