What do you mean by make the DDR3 perform more like DDR2? All modern DDR3 modules are faster than any DDR2 modules so the point seems moot. Also, you can only install DDR3 memory on a motherboard and CPU system that supports it. Your motherboard probably doesn't because I think there aren't any Xeon boards that support DDR2 and DDR3.
DDR3 and DDR2 are different generations of the DDR (Double Data Rate) technology. They are not compatible at all with each other, never have been, and never will be. A memory module is either DDR2 or DDR3 or a different memory tech but no modules support multiple technologies nor different generations of the same technology.
DDR3 memory modules have a different pin-out then DDR2 even though they have the same number of pins (240, 120 per side of DIMM). A few motherboards have some DDR2 and DDR3 slots but they are all old consumer motherboards.