I considering installing Devuan, but I want to know if it is possible to install it with only free firmware, and after installation, adding a specific non-free firmware that will make my wifi to work.  I want to be the one to supply this wifi firmware, not Devuan to install it automatically during installation.

Let me explain, I've got a new machine, and currently Debian is installed in it.  It uses iwlwifi kernel driver for the wifi to function.  This driver in turn needs non free firmware to function.  Listing the files from /lib/firmware, I see lot of files beginning with iwlwifi-*, so I assume these are the firmware files that iwlwifi needs.  I want to copy these files in pen drive.  Next, after installation of Devuan with only free firmware (this will not add non-free-firmware section in /etc/apt/sources.list I assume), I'll add these files back from my pen drive.  Of course, these files will not be updated.  I would also like to use these same firmware files in future versions of Devuan (like 6, 7), will they still work?

Currently, Debian tells us to use firmware=none during installation, to prevent installation of non-free firmware.  But, some people from Trisquel forum noted that, passing this option would also prevent installation of free firmware.  As Devuan inherits from Debian, I'd like to know if Devuan inherits this anti-feature too.  The only information I have is from How to install Devuan page, which just says to choose Expert Install. 

So, this is my plan, and I want to know if this is possible.  And yes, I understand the security implications of all these.