I have never seen or heard of metal conversion assemblies. The plastic ones are offered at the below link.

http://www.amazon.com/Dimension-Inst...556438&sr=1-31

I have used both the PD-A530 and PD-M324 combined SPD and platform pedals and personally prefer them to the Dimension pedal inserts which raise the feet enough so ideally the seat height should be readjusted. Also the Dimension pedals so far as I know are only available in one size so that your feet, depending on foot size, may not be properly positioned. When installed the pedals also hang pretty much upside down, IMO worse than having to flip the SPD/Platform pedals to choose sides.

Due to the nature of any insert pedal converter I would expect it to also have the disadvantage of raising the shoes enough to cause a need to readjust saddle height.