Youtuber Orvis25 suggested that I should make an addon for this mod so I made it :) Just put the gamedata file in your game file and done!
With this addon, NPCs will now drop artifacts! Each kill will have 1% chance to drop an artifact.
But according of my knowledge about the files, it seems novice and other low ranking stalkers won't drop artifacts, the higher ranking stalkers will have better chance to drop artifacts, so some artifact's drop rate will be around 1.1% - 3%, according their ranks and factions.
I can make the low ranking stalkers also have a chance to drop artifacts, leave a comment below if you want it. :)
changelog 07/11/2018: With using the optional file, Npc will now drops old ammo. Suggested by Orvis25!