I also have that Macbook Pro (15-inch, 2017, 2.2Ghz) and a non branded USB 3.0 ExpressCard with the NEC UPD720202 Chip. I had been able to install it in my Mac some time ago following instructions found on web, but it stopped working as soon as I upgraded to El Capitan. Now I decided to give it another shot, as some time has passed by. So I found some potentially useful information in some in "hackintosh" related websites and I believe I was able to get it to work again by following the instructions on this site:
Basically, it implies that we give up on some important Apple's security measures by enabling the "kext-dev-mode", typing this command in the Terminal:
sudo nvram boot-args="kext-dev-mode=1″
You may also need to get an updated kext file. That page is providing the download link for one that should be compatible with El Capitan, and maybe you can find some other option in tonymacx86.com.
You also need to disable System Integrity Protection. The instructions are here:
It consists in entering Recovery Mode (CMD+R right after turning on your Mac) and typing this in the Terminal:
csrutil disable; reboot
Obviously, I would advise you to first start by making a full backup of your files and also by reading those articles and try to understand the security/privacy implications of each step and consider you you are willing to take the eventual risk. I would try to get those two security measures back in place and see if the ExpressCard keeps working as expected. That's what I intend to do in my next reboot, when I get my current backup done. ;)
Update: I have just re-enabled System Integrity Protection by using the shell command
csrutil disable; reboot in Recovery Mode. It seems to be working just fine.