I need to install some packages to add a NIC driver to my kernel. Because I don't have it yet, it's not possible for me to do this from my system. What is the best way to solve my problem?
You can download the .deb files and install them manually, but without a connection you won't be automatically getting dependancies and updates. That would be like going back to Linux pre-package manager days, about like... 20 years ago. Perfectly possible, but much easier to get the computer connected and updated/installed.
You can also have a local repository that you take off-line when needed, for example apt-mirror.