How to I apply a 'standard clearcoat 30 sheen' to some unfinished hard maple
I would not get your hopes up of getting an exact match. 

According to the specifications on their site for that part:


  We finish all our tread materials (anything you’ll step on) with a Conversion Varnish. This is the type of finish used in the hardwood flooring industry, it’s strong and designed to take lots of abuse. It’s nearly impossible for a homeowner to achieve this level of quality, because conversion varnishes must be sprayed on, and they are only sold at specialty stores.


Looking around online, there are several conversion varnishes available (e.g., General Finishes), but finding them may be a bit difficult. General Finishes lists it as being sold at Rockler, Woodcraft, and other woodworking stores, but it does not show up on those websites. I doubt they'd have them in-store if they're not available online. Varnishes from other manufactures may be more available, but you may have to visit a specialty painting / finishing store (i.e., not the blue or orange big box).

If you are able to find the conversion varnish, you will need some spraying equipment, like an HVLP system, as well as a catalyst for the varnish. The "30 sheen" is a satin finish.

On the Product Q&amp;A for the stair, they say:


  You can order a finish touchup kit with matching stain &amp; clear finish spray can. This is currently not available on our website, but can be ordered over the phone.


in response to a question about filling nail holes. You may be able to get those cans, but I'm not sure they would be appropriate for anything more than touchup work. You can certainly call them to be certain.

You could use another type of clear finish, like a lacquer or wiping varnish. Those won't have the durability that conversion varnishes have (probably not an issue for stair risers), and there may be differences in the look of the finished product. Given that you're doing it yourself, though, you should expect differences in look even if you do use a conversion varnish. If you go this route, finish selection might be better asked at the woodworking stack.

You could also take them to a professional and ask if they could do it.