Hello Andrew, thanks for your answer.
You are absolutely right - when creating models in modo/blender, exporting them to fbx and importing them to UE4 everything is fine 
However, i collected some scenes that i only have in Unreal without the âoriginalâ 3d model files. Those are mostly bought unreal projects. Some of the contained models have tons of details and a far too high polygon count. Those are the ones i wish to edit.
FBX obviously does not support material instances, but i still was confused that the exported fbx (exported from UE4 to fbxâŚ) only contained âBaseMatâ instead of the actual material names - which seemed like a bug to me.
Since there seems to be no other way, i will start âfixingâ the fbx that i get from UE4.