First a word about definitions. DDR is short for DDRSDRAM, and means Double Data Rate Synchronous Dynamic Random-Access Memory - so it is already SDRAM.
I assume you want to substitute it for older single clock edge SDRAM. The simple answer is that you can't. Primarily, because the DDR-SDRAM is manufactured mostly as 2.5 volt memory. The older SDRAM is manufactured mostly as 3.3 volt memory. If you applied the higher voltage to the DDR memory, it would overheat and fail.
There are other problems with the substitution such as timing, pinout, and a different number of pins on the module edge.