P3D dds files to work in FSX Steam?

I have Milviz Porter Steam version and as usual searched repaints for it. One of the most useful for my purposes would have been Jankees' Australian Army version and the Talair colours. Pity they don't show in FSX. The German EDRM guy altered his P3D repaints to fit the FSX too. So, what's the trick? Both P3D and FSX paints have same dds files and same names. Only difference I've noticed is the size. The original paints that come with the plane amount to 30mb and the hybrid skins to 200mb each. I have Photoshop and basic computer skills...