For example, for "ArenaTransition", the game applies 2 textures on a surface (ArenaConcretePxz and ArenaTransition overt it)
ArenaTransition acts like a decal to dirt the wall.
I never tried but maybe it's possible to add some lines on the NadeoImporterMaterialLib.txt with 2 custom textures.
If someone have some free time, he can try

Edit: I tried but not succeed (with custom textures and nadeo textures)