When you start a new painting, or open the paper settings panel for a layer, at the bottom of the panel you'll see a row of circular controls. At the far right is color, and at the far left is the grain preview. If you click in the circle that has the grain preview it will open the paper texture panel. You can either choose one of the three defaults, or you can click on the 'Load a file...' button.
That will allow you to load your custom paper bitmap.
When you've set up the paper's colour, roughness, metallicity etc. you can click the 'Make New Preset' button to add that canvas to your presets selection.
AndyRage's mantra for graphics engine code:
"Sure - how hard can it be?"