Hi,

There are multiple ways how you can do the conversions. Here is my version.
Guide to understand the workflow: http://www.marmoset.co/toolbag/learn/pbr-conversion

UPDATE! I simplified the mental ray and the vray conversion trees.

Mental ray versions MILA and mia_material (updated):
https://dl.dropboxusercontent.com/u/83350004/metalic_to_spec_mr_mila_and_mia_maya2016.zip

Vray version (updated):
https://dl.dropboxusercontent.com/u/83350004/metalic_to_spec_vray_maya2016.zip

Textures generated by a substance (update the original plug-in!).
Download this great substance:
https://share.allegorithmic.com/libraries/906

Detailed explanation of the shader tree and the workflow (mental ray MILA):

• Before everything: update the Substance plug-in (only the updated version supports new PBR based .sbsar files – created with Substance Designer 5 or above) and use my color management fix for the plug-in (unless you want to add gamma nodes manually – noooooooooooo).
  In this case my fix is not necessary because we won’t use substance auto bake features, but it’s a good thing you have.
  Off course use Maya’s great Color Management System.

Preparations:

• Create a Substance node, load an .sbsar file. I used this great substance https://share.allegorithmic.com/libraries/906, created by SashaRX.
• Turn on Albedo (base color), Normal, Roughness, Metallic inside the substance.
• Create 4 file nodes in Hypershade.
• Delete the automatically generated PhongE material.
• Connect the substance generated nodes to the file nodes:
  (Substance node: Out Image output goes to file node: File Texture Name input.)
• For Metallic, Roughness and Normal file nodes change the Color Space setting from sRGB to Raw. Leave sRGB for the Albedo (Base Color)
• Connect the place2dTexture node to all file nodes. (Drag Ctrl+MMB, this creates the default connections between nodes (if it is possible).)
• Turn on Alpha is Luminance in Metallic, Roughness and Normal file nodes.

Diffuse conversion:

• Create a Multiply node, connect the Albedo (base color) file node (Out Color) to Input 1
• Create Reverse node, connect the Metallic file node (Out Color) to the Reverse node.
• Connect the Reverse node to the Multiply node as Input 2. Make sure Multiply node Operation mode is set to Multiply.

Specular conversion:

• Create a layeredTexture node. Create a second slot. Set the right slot color to 0.040, make sure the alpha value is 1, blend mode is none.
  Select the left slot. Connect the Albedo (base color) file node to color.
  Connect the Metallic file node (Out Alpha) to Alpha (left texture slot!).
  Make sure the Blend Mode is set to Over.

IOR conversion:

• Create a Ramp (texture) node (delete the additional palce2dTexture node you don’t need that).
  Connect the Metallic file node (Out Alpha) to the Ramp (V Coord, because it’s a V ramp by default).
  Set the two colors of the ramp. First slot: set to something around 1.45 (not metal), second slot set to something above 13 (metal).

Almost finished.

MILA connections:

• Create a MILA material. Leave its Base Component as Diffuse.
• Connect the bump2d node to Overall Bump (created by Substance earlier).
• Connect the Multiply node to Color (at Base Component)
• Create a layer (+ Layer), choose Fresnel Layer, choose Glossy Reflection.
• Connect the “IOR ramp” to Fresnel IOR (simply MMB drag, release above the attribute).
• Connect the “Specular” (layeredTexture) node to Color (Glossy Reflection layer).
• Connect the Roughness file node to Roughness (Glossy Reflection layer).

That’s all.

The final tree:

Cheers, D