Nothing is impossible, however the benefit of taking on a project like this would be nowhere near the amount of work to do so.
The MacBook Pro Retina has multiple ports (MagSafe, Thunderbolt, USB, SD) on each side, whereas the MacBook Pro only has ports on it's left hand side. The battery connectors are quite different and a battery transplant would be VERY difficult thanks to the highly proprietary design of the MBPR's battery. The majority of screw locations are different. The list goes on. For comparison purposes:
MacBook Pro Retina internal view
MacBook Pro internal view
It would be kind of a cool transplant but in reality it just isn't feasible.
Hope that helps!