To create a custom preset, Substance Designer is needed. 

The creation of custom preset respects the same rules as creating a custom filter for Substance Alchemist. Documentation here.

Creation

Creation of the graph

Open Substance Designer and create a new Substance.

Open the graph properties and fill the following mandatory information:

• Label: Enter the name of your custom preset that will be used in the Substance Alchemist interface
• User Data: Add alchemist::type=filter

Definition of inputs and outputs

Inputs

The inputs represent the material channels you want to transform before the export.

Create Input Color node (or grayscale) per material channel and add a usage in the attributes to each input node to ensure the connection is made between your material(s) and your custom preset.

Example: Definition of the Base Color input

Outputs

The outputs represent the result of your texture export.

Create Output node per texture and add usage and a label in the attributes to each output node. The label will be displayed in the Channels list in the Exporter window and in the name of your texture file.

Example: Definition of the custom texture Color Opacity

Example of channel packing and channel conversion

Packing of 3 grayscale channels in one RGB texture:

Channel conversion from PBR Metallic/Roughness to PBR Specular/Glossiness:

Import

In Substance Alchemist, you can import your custom preset directly in the Export window. Open the preset drop-down list and click on: Import presets

You can also click on Manage presets and at the bottom of the window, you have an Import Preset button.