Problem solved.
It seems that Radiant does have a directory for these, however, in my case, it wasn’t extracted.
In case anybody comes across this problem in the future, I will detail the solution below:
In the folder that you pulled in the gamepack (games/ETPack) from SVN, you will find a handful of pk3s (common.pk3, mapmedia.pk3, lights.pk3, and astro-skies.pk3), I made a temporary folder inside of the ETPack directory and unzipped them all. You will then have a textures folder, I then copied the contents over to /usr/local/games/enemy-territory/etmain/textures/…or wherever else you specified your EnginePath to be.
You will also find a few more folders in this directory, I copied them over to etmain also, specifically models.
Hope that helps! Thanks for your replies above ^^