Hi,
It would not be possible to convert materials to legacy mtl and there will also be no such feature in the future also.
But if you have a material that has IOR with a value of 3 or more, they will be automatically converted to metals. In case, you want to change metals to non-metals then you can use this simple script to batch change material type from metal to non-metals.
PS: You should not run this script in your whole 3ds Max project otherwise the materials which should be metal will also be converted into non-metals. To use this script, simply select the offending materials which are supposed to be non-metals in a separate file then run this script and after that, you can merge those 3D objects with correct material properties with your original scene.
I understand many people might have problem with this approach but this is the best workaround currently to fix your wrong metal materials. We are continuously working on improving the converter with upcoming releases.
Regards,
Avi