Use Regrain to add grain from the RGB channels of a selected colour in an image.
To access the Regrain menu, use:
Batch, then select a node from the Node bin.
Timeline, then use Batch FX (see Creating Batch FX).
Tools, then select from the menu (See Accessing Tools).
The Modular Keyer, then select a node from the Node bin.
This node accepts front, back, and matte clips as input, and outputs a result.