I think you're referring to
ReadyBoost. That's a different thing: a cache for hard drives other than SSD. If you mean using USB for pagefile.sys (i.e. moving the virtual memory), that's also possible. My experience has been it's usually not worth the effort, but sometimes it is :-) But with USB 3, maybe it's worth a revisit and I may fart around with this on one of my slow machines.
But you can't stick a thumbdrive in and simply extend the addressable system RAM with some voodoo-like incantation.