Hi there,

Here some help. 🙂
Maya 2016 files.
Two for mental ray and one for vray.

Contents:
sphere test object,
Shader and the conversion tree (File nodes for Base color, Roughness, Metallic and Normal)

https://dl.dropboxusercontent.com/u/83350004/PBR_converters/metalic_to_spec_mr_mia_empty.ma

https://dl.dropboxusercontent.com/u/83350004/PBR_converters/metalic_to_spec_mr_mila_empty.ma

https://dl.dropboxusercontent.com/u/83350004/PBR_converters/metalic_to_spec_vray_empty.ma

How to use:
– Import the file into your scene.
– Load the textures, or connect the substance nodes. (outImage to fileTextureName)
– Connect the place2dTexture to every file node (default connection: drag with Ctrl+MMB).
– Mental ray: Make sure you always set the non color input textures types (normal, bump, roughness, IOR, etc.) to RAW (instead of sRGB) and use the color management system of course.

Cheers, D