You can BIY your own Fusion Drive on your Mac.
Lets hope Apple bring this into the GUI and supported under OSX. Seems like they might not as it’s a support nightmare and not many Macs official support that hardware config.
I’d have to ditch the DVD drive in my 13” MacBook Pro to make one of these and I’m just so busy right now I cant see that happening.
I’ll probably upgrade before I get around to it, or go pure SSD. I think Apple would prefer people upgrade hardware to get this feature rather than retro-fit it to an existing Mac.