I also wanted another shelf. ClosetMaid's Web site showed an “Extra shelf kit 34.75 x 17.85 inches” but this is not available anywhere that I could find. Consequently, I was compelled to build my own shelf. It wasn't difficult, but took some care. Previous wood craft experience was helpful.
After looking at various options, I settled on a pine laminated project panel as the best choice. HD offers an “edge-glued panel” (sku 493562) but this is 6 feet long and I only needed 34.75 inches of this. From another source I found a “Craft Master's Lodgepole Pine Laminated Project Panel” that was available as 48 x 20 x 3/4 inches, readily cut down to 34.75 x 17.85 with less scrap.
Given that this material was basically a series of pine boards glued together (albeit well glued), I decided to reinforce the shelf by adding 3/4 inch aluminum angle stock to each 17.85 inch edge. I cut the long dimension of the pine project panel slightly short so that the shelf with the angle stock pieces added was 34.75 inches. I secured the angle stock in place with countersunk #6-1/2 inch flat-head screws along the edges, and #6-1/2 inch pan head screws along the surface part. (Also note, cutting this shelf accurately was important so it will fit correctly and not fall off the shelf supports later.) Prior to mounting the angle stock pieces, I coated the entire surface of the shelf (including the edges) with clear polyurethane for protection.
Finally, I fashioned four new shelf support pieces from 1 1/2 inch lengths of rectangular molding, 1/4 x 1/2 inches. I had some pine molding on hand, but if I were doing this again I would try to find a denser wood. As it turns out, the shelf support holes provided in the cabinet walls by ClosetMaid have the correct tight clearance for metric M5 machine screws. #10 screws may also work, but are slightly smaller and the M5 screws fit more snugly. I drilled holes in each of the molding pieces to provide for a tight fit of 2 cm long M5 machine screws. Tricky part here is that the four shelf supports must be identical, so the holes for the M5 screws must be carefully drilled. Best to use a drill press and vice so these holes are identically placed and true. This solution worked well, but it might be better if you can find ready-to-use shelf supports - Not sure where to find these.
I debated painting the shelf white, but in my application this was unnecessary.