I have these same shelves, and have modded mine to take extra shelves. (see pics)
First I got a bunch of these:
Should be able to find in any hardware store. they are cable clamps. Need I believe the 5mm ones.
I also bought a bunch of this:
https://www.homedepot.ca/en/home/p.std-hardboard-panel-14-inches-x-2-feet-x-2-feet.1000434567.htmlCan also get in the 2 feet x 4 feet.
With the boards, you will need to cut yourself, or can get cut at Home Depot or other stores to the following size:
13" x 15"
As can be seen in SHELF 1 pic, I like to double mine up (glued) for added stability.
Finally, I recommend getting some heavy gauge wire, usually find that at a hobby store, as they don't normally carry that in Home Depot.
With that, I like to make a few support bars as well as a cross bar to rest under the shelf. Again, these are me being extra cautious to ensure nothing falls (see 3rd pic).
all told, the costs are extremely cheap per shelf for extra selves, and should cost around $20-25 to make around 4 shelves.