Is it possible to enable "Enemies never attack first" option in an existing save game? Could I modify a specific file to achieve the same?
You can use this command to turn peaceful mode on:
/c game.player.surface.peaceful_mode = true
You can open the console by using the ` key.
Peaceful mode will make it so enemies will not attack you unless you attack first. Checkout the Wiki for more commands. Note that this will disable achievements.