Hi I'm new in the speedruning community, and I would like to know if was possible to modify the rng file of a game to have the best possible rng in a game for every runs? I know that's it can be done in-game and that's legit, but I wanted to know if I could do it Before starting of a run? Thanks! :)

That would depend on the game and the community for that game as they all define their one rules. But in general, modifying game files of any kind is usually banned universally as that is considered cheating. But, the only definitive answer is to check with the community for the game in question.