tried to answer on the Steam thread, but you deleted it. so here it is.
Importing of a painted terrain map is not currently possible. The painted map is a texture that is painted with different alpha for the different textures on it.
the map size is 51000 x 51000 so it would be possible to just have a flat map and make one big decal over it the way you want is by painting it in a paint program and applying the texture to a flat plane mesh and scaling it the size of the full terrain. I believe there is a statement that can be put in the FPE file to stop the lightmapper from having a fit cause of the size.
I will check on that. But us you want to experiment, then I would have the texture size at least 4096x4096, probably a bit of sharpening as it will be stretched over a large area. Maybe even do this in 8 of these to cover the whole map and get a better resolution on each one. Make sure that you turn off physics and have a collision mode of 0 in there FPE files.
Not the best solution but a small amount of polygon as each of these planes has only 1 polygon in it.
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