I've had all my door panels open from time to time to put in new speakers, pop out dents, replace power window motors, etc. The rear windows can't go down any further than they already do without extensive modification.
Here is how the power windows work. On each side of the window, there is a track which keeps the window aligned. Then in the center there is a regulator and motor. Here is what it looks like:

On the left you see the motor. On the right you see the regulator. On the middle of the regulator you see the piece that attaches to the bottom of the window. This piece is attached to a cable which the motor pushes and pulls. When this piece reaches the bottom, the window cannot go further. I guess if you had access to a fantastic metal and fabrication shop, you could lengthen the regular. But then you would need to lengthen the cable and the alignment track.
So to put it short, no it is not possible.