How do I get the calculator button on a Microsoft Sculpt keyboard working?
If macOS isn't "seeing" the scan code then there's not much that can be done in mapping it - it's got to see it to map it.

There is a utility you can use that can verify if the keyboard is sending the code or not - xev.  You'll need to run it in XQuartz which is an X11 environment.

Using the following command, you can get the output of each keypress 

xev | grep -A2 --line-buffered '^KeyRelease' \
| sed -n '/keycode /s/^.*keycode \([0-9]*\).* (.*, \(.*\)).*$/\1 \2/p'


Below is some random keypresses on my wired Unix keyboard attached to my Mac.


I've used this to test and map Sun Microsystems Type 5, 6, and 7 keyboards on my Mac and can verify that it detected the extra keys.  If xev can see it, it means that the scancode is being sent, it's just the HID kexts macOS uses are insufficient.  

Can it be fixed?

That would be a software development question outside my and this site's scope.  

Bottom Line

If it's not being detectable at all, it's unusable.  If it is being detected, there's a chance of using it, but you'll need an application to see the keypress to utilize it.

(You might want to try ControllerMate)