Some motherboards support both. But there must be specific support for both types. If not then the answer is unfortunately no. You'll need to check your motherboard manual for more information.
No. You cant put a DDR2 in a DDR3 Slot, even if you try....it doesnt fit at all. The speeds are diferente, everthings is diferent. There is some motherboards that actually have EXTRA memory slots to put DDR2...but its uncommon. DDR3 is very cheap...go buy some memory.