When you are designing a device with a CPU that only supports ddr3, is it possible to use ddr2 in that design instead?
You can't (*). 

DDR3 is not backwards compatible with DDR2 and vice versa. You can't attach DDR2 RAM and expect a DDR3 external memory interface controller to work with it.

Your best bet is simply to design around DDR3 and update your design files.



(*) You could get an FPGA and write some controller to allow it to appear as DDR3 memory, and have it interface with the DDR2 memory as effectively some form of translation, but that is really a pointless idea as it would take far more time, effort, and cost to do compared with just designing a DDR3 PCB footprint.