I've actually done this. The front seats can be pushed forward, but they don't fold enough to put a sleeping pad on top. I stacked duffel bags to fill in the floor space, but I've seen people use coolers or big plastic storage containers too. I found it way more comfortable to sleep with my head at the rear of the vehicle.
It's a tight fit for a short person like myself (I'm 5'6"), but it is possible.
This guy has a really good setup for his Prius if you're looking for further research: