Can you tell me why it is not possible ?
but more seriously, even the fastest of SSDs cannot stand up to the performance requirements of Random Access Memory. Everything in RAM needs to be available at any time.
But wait! PCs use such a thing all the time!
The use of Virtual Memory/swap (what you're proposing) can potentially be a plus, having done stuff like that before on my iPod touch, but will all but certainly result in varying degrees of degraded performance, lag, and wear on the memory card it was used on. (Really depends though)
I guess from a cost-benefit analysis, such a feature would add complexity, risk wrecking the performance WP is known for if not done carefully, take already limited resources from other more pressing projects, add another point of failure, and will likely shorten the end-user's SD card's life down the road.
And for what benefit? Multitasking could improve, I guess if some lower priority stuff can be moved to the card, heavier games might be possible.
I don't think my writing here is exhaustive, so feel free to correct and add. Just my thoughts.