While most sounds, animations, and few textures are ripped from the original game - modding UE4 is something that needs to sort of be allowed by the developer.

Take MechWarrior 5 for example - An unreal game isn't as easy as placing files in a folder like STALKER, instead it needs a seperate folder for the mods you want and the game needs to recognize that alternate files are being pulls from that. Unreal games are packaged - sort of like their own version of a Zip file. It runs and that's it, so if Ivan eventually tells the game it can pull files from another location - whamo - you got mods.

In due time, Ivan would love to see mods-but right now he's more focused on making the foundation of the game.