I assume you just want to fill in the empty screw holes, right? I would warm the riser, to help imprive the absorption, then vacuum the holes and drip superglue into the holes. Just a couple drops each, it should just soak into the wood. If it bridges the hole, or covers it, then just poke with splinter to break the bubble.
Next day - warm riser again and fill holes with epoxy. I would mix epoxy on a warm surface so it is thinner, then draw into syringe and squirt into the holes. Might be tough if the holes are real small. If too small then you might fill with superglue.
You might want to add a little sawdust of the right color to the epoxy while mixing.