No, it doesn’t work that way, it wouldn’t have anything to do with UE4. Speedtree is a middleware that can create trees and other types of vegetation for games, UE4 and many other engines support it. The .srt file is something generated from the Speedtree software and if you find .srt files in Witcher 3 then most likely they are compiled in a way (for the shipped game) that are not editable.